Bug 796729 - web activities cleanup

This pull request makes little changes to a lot of files, to tweak the names and types of activities used throughout Gaia.  The activities we've made up for Gaia are a public facing API that third party developers will use and we'll have to support going forward, so this is an attempt to clean them up a bit.

See the discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=796729 and https://etherpad.mozilla.org/gaia-activities-audit