Fix: Add header to gitlab.js

[glensc]

🌟 What does this PR do?

Fixes error reported by CI:

• [gitlab]: Fix selector for sidebar time-tracker class #2156 (comment)

reverse-engineered update-origins.js to understand what the hell the error means, as there's definitely nothing written about this in docs/CONTRIBUTING.md

[glensc]

this line was automatically added by update-origins.js script.

[glensc]

seems to me that CI job running update-origins is half-baked, existing integrations were not modified to add the header. also there's nothing documented about format of the header in CONTRIBUTING.md, and the update-origins.js itself has 0 comments about logic or what should it be doing, some obfuscation contest candidate there.

[glensc]

Should be rather trivial to write a script that:

1. takes current origins.js
2. assembles expected comment block for each integration
3. writes that to each integration replacing existing block if it's there.
4. git add -p to verify each change
5. commit the changes

but given past slow and negative experience with this repo, I would not do that unless someone explicitly asks to do that.

also, the error that the script outputs, should guide the user to go to docs/CONTRIBUTING.md#file-header, and that # File header block should be written first.

[glensc]

Also, CI for update-origins is set up incorrectly. it runs in the context of the contributing user repository:

• https://github.com/glensc/toggl-button/actions/runs/3960074160

so, it doesn't appear at all under pull request:

• https://github.com/toggl/track-extension/pull/2158/checks

probably another reason why other integrations lack the header. I just happened to read my email to see the CI failure.