Added application for external links.
Fixed and updated schemamigration.
Merge pull request #143 from teemulehtinen/services

Moved LTI Services under External Services and added external Link Service
Merge branch 'master' of https://github.com/teemulehtinen/a-plus into ext

Conflicts:
	settings.py
	templates/course/_course_menu.html
Merge branch 'ext'