chore(ui): always use primary button color inside modals (#7161)
Followup to https://codeberg.org/forgejo/forgejo/pulls/3002
Related to https://codeberg.org/forgejo/forgejo/pulls/6126
This deimplements `ModalButtonColors`.
Nowhere in the UI except for one devtest page modal dialogs paint this button any other color and we don't want new UIs to do that.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7161
Reviewed-by: Gusted <gusted@noreply.codeberg.org>