DM creation: opt-out for e2e in the UI

[manuroe]

From riotX-android created by bmarty: element-hq/element-android#837

With possibility to opt-out.

Screen to update (create DM screen):

In this screen we should have somewhere a check box, checked by default, to enable encryption for the room the user is creating.

Also we should consider to update the create room screen, to add a check box, not checked by default, for the same purpose:

@nadonomy could you propose some UX, especially for the create DM screen. It's more straightforward on the create room screen, but feel free to propose something too

[manuroe]

Moved to the next sprint. The first step consists now in enabling e2e automatically with no opt-out UI: #2943.

[manuroe]

Postpone this UI change for that for now.
E2e is always by default in DM.