Make `add line comment` buttons focusable (#25894) (#25896)
Backport #25894 by @sebastian-sauer
Use a real button and add an aria-label.
Additionally, show the button whenever it is focused.
See https://codeberg.org/forgejo/forgejo/issues/998 for explanation.
Our handling of this button is now equal to that of GitHub.
Nothing has changed visually.
Co-authored-by: sebastian-sauer <sauer.sebastian@gmail.com>
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