Cuased by: https://gerrit.wikimedia.org/r/#/c/184280/

Steps to reproduce:

1. Open a page (no matter if stable/beta/alpha or if the page already exist)
2. Click edit pencil
3. Type in some text and click next
4. Click save

Expected: You see a "Save, please wait" message and a spinner and you will be redirected to the page after the edit was saved
Observed: The edit is saved correctly, but the user hangs in the "Save, please wait" screen and get's no info that the page was saved or what he has to do now

Error in console:
Uncaught TypeError: Cannot read property 'always' of undefined
https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/javascripts/modules/editor/EditorOverlayBase.js#L166

The problem is not branched in a wmf version actually. It will be in 1.25wmf16, so let's fix it before that and we save a SWAT :D