Bug 847168 - [User Story] Email Signature

I feel like we should do a prompt here if we notice the text has changed and the user tapped back, similar to what we do for the Compose card. My first attempt at editing, I just edited then hit the back button, since spatially, I knew I was going to the left back to settings. But I lost the changes.

We do not have to be super fancy here and detect if the card is hidden though. Signatures are short, and it should be enough to just detect if they do a quick edit and hit Back instead of Done. We still need to prompt though (and just not auto-save) because we need to give a user a way to cancel the edits if they decide it was not what they wanted.

We should confirm this with UX too, as part of ui-review, but I am tending towards expecting that they will want to not lose the user's work. Depending on the time scale for this ticket, we can also just ask in the Monday meeting, maybe even do a ux walkthrough with them to get to ui-review+ if we have the dev changes done.