[web] Provide all modal dialogs with loading spinners

[hurradieweltgehtunter]

All buttons, which confirm a form or some sort of dialog, should receive a spinner, which indicates that the desired action is running. This applies to:

— Modals
— Forms, for example sharing dialogue
— …

Current state leaves the user often with an unresponsive UI, which seems to be doing nothing or broken.
The user is tempted to reload the page, which cancels the action running in the background and leads to unwanted results.

Related:

• [files] Batch actions need some kind of working in progress-feedback (loading spinner) #9182
• Modify dialogs to use <form> elements #9018

For modals:

• show loading indicator in submit button if action is in progress (same as in sharing when inviting users)
• Add an offset after which the indicator appears (maybe 700ms as in sharing?)

visual

loading indicator like we have it already in the share button

[kulmann]

On top of that we also really do need to prevent submitting the respective action multiple times, see e.g. #9189

[hurradieweltgehtunter]

On top of that we also really do need to prevent submitting the respective action multiple times, see e.g. #9189

I think this is also connected to #9018