[DIFE-793] Changes to the create event form

Here is the list of the changes that need to be done to the event creation form:

Remove/hide option “In this platform“ from the “Registration Type” field - just for the user-generated meetings;

The Official events should still permit the admins to create meetings with the Registration type - In this platform.

Remove/hide field “Organise event as“;

Set as mandatory the field “Online meeting URL”, when a user selects as “Type” the values “Online” or “Both”,

Set as mandatory the field “Registration URL”, when a user selects as “Registration type” the value “On a different platform