Convert ref dropdown to Vue [RUN ALL RSPEC] [RUN AS-IF-FOSS]

MR	Changes
This MR	Mount simple vue dropdown
TBD	Connect dropdown with Vue router
TBD	Add search to dropdown
TBD	Update feature spec and ensure FF coverage
TBD	Clean up: Remove Project.js

What does this MR do?

This MR series is an effort to convert the ref dropdown from HAML to Vue. This feature is built behind a FF, to allow us the flexibility to build it iteratively and in favor of smaller MR.

This MR is focused on mounting a simple vue dropdown in place of the original ref dropdown. Note: the dropdown won't actually update anything. That will be addressed in a follow-up MR. In this MR, we're just focused on the UI.

Testing case

This feature is behind a feature flag, to turn on:

rails c

Feature.enable(:vue_ref_dropdown)

1. Navigate to Repository > Files
2. You will see that the ref dropdown is using our PJ dropdown component

Close	Open

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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

Related to #327085 (closed)