Global Search - Header Search Default 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 2nd in a series for a full refactor of the deprecated component mentioned above. Here we are adding in the start of the Vuex store as well as the default dropdown items.

IMPORTANT: No search functionality is implemented in this change. Also no a11y functionality is being considered in this change. Only that the dropdown renders and the links are pointing to the right place.

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. Ensure it opens up a pre-loaded dropdown with the title "All GitLab"
4. Click each link and ensure it takes you to the correct Issue/MR filter without a Project or Group filter

Group

1. Navigate to your Group based GitLab route (ex. http://127.0.0.1:3000/flightjs)
2. Click the header dropdown
3. Ensure it opens up a pre-loaded dropdown with the title the same as the name of the Group
4. Click each link and ensure it takes you to the correct Issue/MR for the specific Group

Project

1. Navigate to your Project based GitLab route (ex. http://127.0.0.1:3000/flightjs/Flight)
2. Click the header dropdown
3. Ensure it opens up a pre-loaded dropdown with the title the same as the name of the Project
4. Click each link and ensure it takes you to the correct Issue/MR for the specific Project

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