Fix comment thread with first comment deleted

[gbalint]

Problem:
When the first comment of a thread is deleted, the comment text in the right sidebar and in the hovered comment marker is empty.

Fix:
The problem is that the deleted comment still exists in thread.comments, it just receives a deletedAt flag.
This means we can not find the first non-deleted comment in comments[0].
I implemented a helper called getFirstComment instead of just using comments[0].

[github-actions]

Try me

[relativeci]

Job #9683: Bundle Size — 62.18MiB (~+0.01%).

69fa020(current) vs df0030c master#9679(baseline)

Warning

Bundle contains 66 duplicate packages – View duplicate packages

Bundle metrics   1 change 1 regression

	Current Job #9683	Baseline Job #9679
Initial JS	45.39MiB(~+0.01%)	45.39MiB
Initial CSS	0B	0B
Cache Invalidation	19.85%	19.85%
Chunks	26	26
Assets	30	30
Modules	4361	4361
Duplicate Modules	470	470
Duplicate Code	30.75%	30.75%
Packages	462	462
Duplicate Packages	65	65

Bundle size by type   1 change 1 regression

	Current Job #9683	Baseline Job #9679
JS	62.17MiB (~+0.01%)	62.17MiB
HTML	11.53KiB	11.53KiB

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View job #9683 report View fix/deleted-first-comment branch activity

[github-actions]

Performance test results: