Make the modal choice between "Edit This Event" and "Edit Future Events" more clear

Summary:
Fixes T11909. Ref T11816. Instead of offering a dropdown with choices between "Edit/Cancel/Reinstate This Event" and "Edit/Cancel/Reinstate Future Events", make the choice more explicit.

This dialog ends up pretty wordy but this edit is rare, so I think that's alright.

Test Plan: {F2046863}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816, T11909

Differential Revision: https://secure.phabricator.com/D16956