Alpha: Add possibility to change font size of the content

Added a fontchanger to Special:MobileMenu for alpha group members,
where they can change the font size from with the use os three
fixed sizes.

The value is saved with LocalStorage (or cookie as fallback) and is set
to the content via JavaScript.

Bug: T50946
Change-Id: I7f4fe3957505ebad0dc17bdf6a99a42d61847283
Merge "Alpha: Add possibility to change font size of the content"