Global Search - Header Search Scoped Items

Intro

This change is broken off from !66007 (closed)
Work towards #297396 (closed)

This change is behind a feature flag (:new_header_search)
Feature Flag Rollout Issue: #339348 (closed)

This end goal here is to replace the large deprecated jquery component used for the Header Search: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/search_autocomplete.js

Replacing it with GitLab UI will allow us to add features more effectively as well as true up the styles permanently by using the GitLab UI component.

What does this MR do?

This MR is the 3rd in a series for a full refactor of the deprecated component mentioned above. Here we are adding the support to the search bar as well as "scoped" search suggestions depending on where in the app you are searching from.

IMPORTANT: No a11y functionality is being considered in this change. The focus here is searching via the search bar and ensuring the scoped suggestions are correct

Here is a full roadmap for when the missing features will be added: #297396 (comment 665578315)

Screenshots or Screencasts (strongly suggested)

Before (No FF)	After (FF)

How to setup and validate locally (strongly suggested)

1. Fetch and checkout this branch
2. From your GDK terminal, access the rails console by typing rails c
3. Enable the feature flag by typing Feature.enable(:new_header_search)

No Group/Project

1. Navigate to your Home GitLab route (ex. http://127.0.0.1:3000/)
2. Click the header dropdown
3. Type a Search Term
4. Ensure the dropdown updates and has one option:
   1. "{term}" in all GitLab
5. Search + Enter Key => Takes you to a search page for your term across all GitLab
6. Click the single dropdown item and ensure it takes you to a search page for your term across all GitLab (Same as enter key)

Group

1. Navigate to your Group based GitLab route (ex. http://127.0.0.1:3000/flightjs)
2. Click the header dropdown
3. Type a Search Term
4. Ensure the dropdown updates and has two options:
   1. "{term}" in group {group}
   2. "{term}" in all GitLab
5. Search + Enter Key => Takes you to a search page for your term scoped to the selected group
6. Click each dropdown item and ensure it takes you to the expected filtered search page (First option should be the same as the enter key)

Project

1. Navigate to your Project based GitLab route (ex. http://127.0.0.1:3000/flightjs/Flight)
2. Click the header dropdown
3. Type a Search Term
4. Ensure the dropdown updates and has three options:
   1. "{term}" in project {project}
   2. "{term}" in group {group}
   3. "{term}" in all GitLab
5. Search + Enter Key => Takes you to a search page for your term scoped to the selected project
6. Click each dropdown item and ensure it takes you to the expected filtered search page (First option should be the same as the enter key)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team