chore: extract FormModal from Modal

[SychO9]

Changes proposed in this pull request:

• The current modal component always assumes the contents are part of a form, and therefore its typings require an onsubmit method be filled.
• The PR extracts a FormModal instead to have a clear separation of read only modals and action modals.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.