- Removed jQuery-Migrate as it seems to no longer be needed.
- Removed a dead code section.
- Fixed some deprecations detected by jQuery-Migrate.

Ref: https://jquery.com/upgrade-guide/3.5/
Ref: https://github.com/gogs/gogs/search?q=repo-name-change-prompt
Ref: https://github.com/go-gitea/gitea/search?q=repo-name-change-prompt
Fixes: https://github.com/go-gitea/gitea/issues/9372

Co-authored-by: zeripath <art27@cantab.net>

Currently, this needs to be its own chunk because fomantic depends
on jQuery being present. The next step is to move fomantic to webpack
too after which we can combine the index,fomantic and jquery files into
one.

jquery-migrate is still neccessary because our ancient version of Dropzone
seems to break without it. I imagine it can be removed after a Dropzone
upgrade.