Add mermaid JS renderer

[silverwind]

For feature parity with GitLab. Tested in files, issues, wiki, editor. arc-green only does an inversion because the renderer seems to like to render white backgrounds on boxes.

Ref: #3340
Fixes: #12307

Previews:

Sample for testing:

```mermaid
    stateDiagram
    [*] --> Active

    state Active {
        [*] --> NumLockOff
        NumLockOff --> NumLockOn : EvNumLockPressed
        NumLockOn --> NumLockOff : EvNumLockPressed
    }
```

[silverwind]

The only real downside I see with this is that the library is a bit heavy at 1.4MiB so will add a bit to our webpack build time. The code itself is rather trivial and should not require much to maintain.

[silverwind]

Took a few hints from the gitlab implementation. Now renders without a border and with neutral theme. Updated screenshots.

[lunny]

This may break the user's customerized mermaid rendering. We should have a warnning for that.

[techknowlogick]

🚀

[silverwind]

This may break the user's customerized mermaid rendering

I don't think it will. Our script will likely run before the user-supplied one and then their script is essentially a noop because it won't find any matching elements.

A mention in the release notes about the necessary customization removal would be nice.

[lafriks]

Looks like editor is now broken with error:

Uncaught (in promise) TypeError: e[t] is undefined
    o https://try.gitea.io/js/index.js?v=2e108e628cde5272a8a152c33efb9249:1
    t https://try.gitea.io/js/index.js?v=2e108e628cde5272a8a152c33efb9249:1

[silverwind]

It does work for me https://try.gitea.io/silverwind/symlink-test/issues/7. Try refreshing the page a few times? There's still a unresolved issue with bad serviceworker cache which I'll soon look into.

[silverwind]

Ah, misread. What exactly do you do to trigger that error?

[lafriks]

Just edit file in repository (I was trying to edit README.md)

[lafriks]

nevermind, now it does work 😅

[silverwind]

Yeah, serviceworker strikes again. I plan to experiment with contenthash chunks which will hopefully bust that cache and fix these issues after frontend rebuilds. Only problem is that we can't use contenthash for the index chunk because the backend doesn't know the filename (could probably be worked around via detection).