Leave fullscreen editor when displaying "Upload File" and "Meme" dialogs

Summary:
When being in fullscreen editor mode, the Upload File and Meme dialogs get opened behind the editor and thus are invisible.
Given the options listed in https://secure.phabricator.com/T8200, pop out of fullscreen when these buttons get clicked, as this is easiest to implement.

Closes T15901

Test Plan:

1. Go to any item which allows commenting, e.g. a task.
2. In the toolbar of the textarea, click the Fullscreen Mode button.
3. Click the Upload File or Meme button in the toolbar.
4. Check if the dialog is displayed.
5. Repeat the test in normal mode (not "Fullscreen mode") and see that it still works.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Tags: #remarkup

Maniphest Tasks: T15901

Differential Revision: https://we.phorge.it/D25764