Bug 937673 - Modify groups model to allow user-created curated groups

* Add flags for functional area, members_can_leave, visible
* Get rid of system, auto_complete, and related fields
Bug 920270 - Basket subscribe API not returning token anymore

The Basket API's subscribe call is going to stop returning the
token and become purely asynchronous. Don't expect the token in
the response, and adapt other code to query Basket for the token
later if we need it.
Bug 938184 - Don't allow creating groups while editing user profile

Part of the changes for curated groups is that groups will only be
creatable from the groups list page, so only allow people editing
their profiles to select groups that already exist.

The tag-it library that was being used to implement the groups
selection could not easily be configured or modified to prevent
users from selecting non-existent items, so the groups field
was changed to use django-selectable. Styling was added to make
the django-selectable field look like the current tag-it fields,
which are still being used for e.g. skills and languages.
Remove some unused code from groups/forms.py
Bug 943736 - Add Jinja tags to access the context
[Fix Bug 932652] Filter out @ and urls from twitter usernames, and validate correct format
Merge pull request #725 from Sancus/932652-twittervalidate

[Fix Bug 932652] Filter out @ and urls from twitter usernames, and valid...
[fix bug 939752] Add Url with text/html.
Merge pull request #706 from dpoirier/bug-920270-basket-api

Bug 920270 - Basket subscribe API not returning token anymore
Merge pull request #737 from glogiotatidis/opensearch-url

[fix bug 939752] Add Url with text/html.
Merge pull request #731 from dpoirier/jinja_context_helper

Bug 943736 - Add Jinja tags to access the context
[bug 699530] Remove taskboard_* tables.

taskboard_* tables are leftovers of now removed code. We removed them
from our database, so now we can remove them from this script as well.
Merge pull request #736 from glogiotatidis/anon

[bug 699530] Remove taskboard_* tables.
merging Bug 937673 - Modify groups model to allow user-created curated groups
merging: Bug 938184 - Don't allow creating groups while editing user profile

Part of the changes for curated groups is that groups will only be
creatable from the groups list page, so only allow people editing
their profiles to select groups that already exist.

The tag-it library that was being used to implement the groups
selection could not easily be configured or modified to prevent
users from selecting non-existent items, so the groups field
was changed to use django-selectable. Styling was added to make
the django-selectable field look like the current tag-it fields,
which are still being used for e.g. skills and languages.
merging:  Remove some unused code from groups/forms.py
Merge remote-tracking branch 'upstream/pr/726' into curatedgroups-merge
[Bug 940941] Show helpful message if searching logged out
Merge pull request #735 from jschneier/issue-940941

[Bug 940941] Show helpful message if searching logged out.
Update migrations