Use mwui Checkboxes on Special:MobileOptions

Use the great mediawiikiui Checkboxes on Special:MobileOptions, instead
of own special toggle buttons.

Add description for option "Images" to clarify, what is meant with this.

Bug: 69487
Change-Id: I7a2e05edfdc8f438c3e7e8a43edc0eeb0122c34d