Add Linear external issue tracker integration

What does this MR do and why?

Add Linear external issue tracker integration

This adds a new integration for the Linear issue tracker. It is a little simpler to configure than the other issue trackers because the URLs can be derived from a simple workspace URL.

References

• Also needs a new logo in GitLab SVGs: gitlab-svgs!1219 (merged) (removed the dependency from this MR, can be re-added as a follow-up)

Screenshots or screen recordings

Screen_Recording_2025-07-18_at_14.43.17

How to set up and validate locally

1. Setup a Linear integration, e.g. with workspace URL https://linear.app/example
2. Link in sidebar (Linear) should now point to https://linear.app/example
3. Using issue reference links in MRs / comments should link them to https://linear.app/example/issue/:id

MR acceptance checklist

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