fix #80: change drag & drop USX file upload validity check

* change check for <usx rather <?xml (not all files have later)
* add notice for invalid file
Merge branch 'master' into feature/admininvite
Fix #81: Project admin can now edit / delete others responses.
fixed #79  Invite a Friend button now only shows with a project context.

API now had logic to deal with getting a project context from the host in the URL, and throws if there is no project context
admin invite wip: finish sendInvite test
Merge branch 'master' into feature/admininvite

Conflicts:
	src/models/commands/UserCommands.php
admin invite wip: connected angular admin invite, stubbed out angular add existing user & add new user