Add form field id generation, remove duplicated ids (#30546) (#30561)

Add form field id generation, remove duplicated ids (#30546) (#30561)

Backport #30546 by @silverwind

Fixes: https://github.com/go-gitea/gitea/issues/30384
On repo settings page, there id `repo_name` was used 5 times on the same
page, some in modal and such. I think we are better off just
auto-generating these IDs in the future so that labels link up with
their form element.

Ideally this id generation would be done in backend in a subtemplate,
but seeing that we already have similar JS patches for checkboxes, I
took the easy path for now.

I also checked that these `#repo_name` were not in use in JS and the
only case where this id appears in JS is on the migration page where
it's still there.

Co-authored-by: silverwind <me@silverwind.io>