Add sort by popularity to issues

What does this MR do?

This MR adds the ability to sort issues search results by popularity (upvotes) and displays the upvotes count on the search result page. This change is based on 2 MRs:

• Add upvotes field to the issues index !64683 (merged)
• Add upvotes_count column to the issues table !65250 (merged)

#263365

How to test this feature locally

Feature.enable(:search_sort_issues_by_popularity)

Migration

Execution time at #database-lab: The query has been executed. Duration: 17.769 min

migration output

❯ VERSION=20210706115312 bin/rake db:migrate:up
== 20210706115312 AddUpvotesCountIndexToIssues: migrating =====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issues, [:project_id, :upvotes_count], {:name=>"index_issues_on_project_id_and_upvotes_count", :algorithm=>:concurrently})
   -> 0.0070s
== 20210706115312 AddUpvotesCountIndexToIssues: migrated (0.0073s) ============

❯ VERSION=20210706115312 bin/rake db:migrate:down
== 20210706115312 AddUpvotesCountIndexToIssues: reverting =====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issues, [:project_id, :upvotes_count], {:name=>"index_issues_on_project_id_and_upvotes_count", :algorithm=>:concurrently})
   -> 0.0081s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- remove_index(:issues, {:name=>"index_issues_on_project_id_and_upvotes_count", :algorithm=>:concurrently, :column=>[:project_id, :upvotes_count]})
   -> 0.0092s
-- execute("RESET ALL")
   -> 0.0004s
== 20210706115312 AddUpvotesCountIndexToIssues: reverted (0.0186s) ============

Screenshots (strongly suggested)

Additional screenshots taken from !65452 (comment 619787282)

Fixed Layout (w/ Popularity FF)

Desktop	Mobile

Fluid Layout (w/ Popularity FF)

Desktop	Mobile

Fixed Layout (w/o Popularity FF)

Desktop	Mobile

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

Related to #263365