Design Management FE to FOSS Part 3/3: Add Designs tab to Issues in FOSS

What does this MR do?

This MR is part 3/3 in effort to move Design Management FE to FOSS (#212566 (closed)).

The primary thing we achieve here is showing Discussion and Designs tab in FOSS, by:

• Updating FOSS HAML to include these tabs
• Moving and updating relevant feature specs (and related code) in light of this change
• Moving Design Management's SASS/CSS from EE -> FOSS

Anywhere that there is a deletion in this MR, there should (almost always) be a corresponding addition somewhere in CE.

~~⚠ Note to reviewers :: This MR depends on all relevant backend code being migrated to FOSS and should not be merged until devopscreate groupknowledge backend team (@.luke and @toupeira) confirm all work is merged, this branch is rebased with master, and we've done some manual exploratory testing of Design Management as if FOSS~~ All related work merged - this MR is ready to merge!

Manual testing tips

• Testing as if FOSS: remove ee/ directory and run gdk reconfigure && gdk restart

  • FOSS users should now have Discussion and Designs tabs on Issues, and should be able to navigate to Designs tab
  • Users should be able to use design management as normal

• Testing as if EE: stock GDK with appropriate license:

  • Nothing should change for EE users

Screenshots

FOSS

In FOSS, we now show the 2 tabs

Before	After

And, when on designs tab (to show that Design Management Vue app is mounted):

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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