Show a warning when editing invitations

[st3iny]

Followup to #3885

Is your feature request related to a problem? Please describe.
After merging the mentioned PR, users will be able to edit invitations again. Those changes will never be propagated to the real event but it might confuse users.

Describe the solution you'd like
Show a warning in the editor that changes only affect your local version of the event/invitation. Only your participation status will be propagated.

Describe alternatives you've considered

• Set the event read only: This approach introduces serious edge cases. If an event is inside your calendar you should be able to edit as pleased. Refer to Cannot change calendar events, in which i was invited by other people #3866 for more information.

Additional context
None

[Bolli84]

@st3iny during reverting it, I recognized the following flaws:

1. The change of calendars only is working in the simple editor, not the sidebar editor
2. The format highlighting (URL now clickable) is not working anymore

[st3iny]

1. The change of calendars only is working in the simple editor, not the sidebar editor

Could you be a bit more specific about this issue? Is this related to #3898?

2. The format highlighting (URL now clickable) is not working anymore

This is working as intended. The url formatter only applies to read only events. After the revert, invitations are not read only anymore so the links can't be clicked anymore.

[ChristophWurst]

Shall we continue in #947?

[Bolli84]

@st3iny
1: in the sidebar, the bespoken event is not editable, in the Simple editor, it is editable (changing calendar).
2: If the formatting is intended seems very weird, however the link is not clickable. not in simple editor, nor in sidebar.

The #3898 issue is also applied here. I only can change the calendar once, then it is not possible anymore.