Fix flash message for flex-container (#30657)
(cherry picked from commit dd2aaadce3ecd3134a1ba0c82c5aaa05d6c11b2b)
Conflicts:
templates/admin/layout_head.tmpl
mostly already done by https://codeberg.org/forgejo/forgejo/pulls/3087
web_src/css/base.css
the conflict is because
https://codeberg.org/forgejo/forgejo/pulls/3350
skipped Remove fomantic menu module (gitea#30325)
and it was not ported.
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