[Flags] Create an event for flag creation

[matteocontrini]

Currently, the flarum/flags extension only has an event for when a flag is being deleted (FlagsWillBeDeleted).

It would be useful to have an event for when a flag is created as well (more precisely, when a user flags a post).

A possible use-case would be, for example, an extension that generates an "external" notification for admins/moderators when a flag is created. (With external I mean an email or a message in a Telegram group, for example.)

Relevant discussion on discuss: https://discuss.flarum.org/d/25981-listening-to-flag-creation

[askvortsov1]

For that matter, it would be nice to update the naming scheme for flags event to fit better with what we're using in core and modern extensions.

[imorland]

I'll happily take this on and submit a PR - this would be very useful for me too..

@askvortsov1 When I rename the FlagsWillBeDeleted, would it be better to deprecate for a release cycle and dispatch the old and new, or just swap out?

[askvortsov1]

Thank you! And yeah we'd want to deprecate it first.

[askvortsov1]

Fixed in flarum/flags#35