flagging improvements, refs #829 and #830

requires forked django-flag app to handle status on FlagInstance
and some admin interface
flagging widget adjustments

- remove inappropriate option
- add placeholder text
- hide widget unless authenticated
welcome email follow up fixes

- fix computation of users to send to
- add site prefix to template
- remove old templates
- tests
Merge branch 'iss-830-flag-improvements'

Conflicts:
	links.py
	templatetags/mapstory_tags.py