Added Activity Modification UI

Added Activity Modification, plus a Rubric Editing screen since it's
related
Merge pull request #69 from graemesmth/master

Added Activity Modification UI
Added Login UI File
Added login screen mockup
Merge pull request #71 from mabalaski/master

Added login screen mockup
Merge pull request #70 from Contramalgra/patch-6

Added Login UI File
Removed placeholder file in UImockups folder