[Feature] Allow more than one comment per line

[delvh]

• Gitea version (or commit ref): 1.14.2 and most current ones
• Can you reproduce the bug at https://try.gitea.io: Most likely, but I cannot be sure as it would be better if it could be tested against another persons comment

Description

Currently when adding a comment to a line in a code review, no one else can add a comment to that line, unless he adds an answer to the already existing comment.
While in most cases that suffices and is wanted, in the case where there are two separate things that should be mentioned regarding this line it will fail. Especially bad because if the answer workaround gets taken, resolving that thread automatically resolves the other issue as well, even though that point may not be resolved yet.

As this is indeed a rather rare case, I'd suggest using a linked comment (comment that refers to its comment below or nil if none present) here instead of a list of comments.

Screenshots

This screenshot shows that I cannot add a comment on that line anymore while one line below (see screenshot below), I can still add a comment: