Refactor forms

[borgoat]

We have a few issues with forms and UX, most likely due to how poorly and inconsistently I initially implemented them...

• In the groups form, the Save button is always enabled, making it unclear whether something happened
• Also in the groups, saving without uploading an image will make the current image disappear
• The creation and update are handled through different paths
• Schedules cannot be updated

There's also the fact that a few forms use this flutter_form_builder library, which is sometimes causing issues, and not really adding much value at this point.

Finally, I have never tested state restoration, focus nodes (for keyboard navigation), and accessibility in general.

I think it would be wise to review all forms and make them more consistent and more usable.