[DB MIGRATION REQ'D] Instance Hashtag DAO
Hashtag, HashtagPost, InstanceHashtag, Post DAO updates
User and Link DAO changes
Enable instance-specific configuration panels
Saved search: Add UI for saving keyword search for Twitter users

TODO:
Add Test of TwitterPluginHashtagConfigurationController (and to
all_plugin_tests)
Make sure TestOfAccountConfigurationController passes
Test what happens to hashtags when you delete an instance user (and
another user has also saved that search)
Add documentation
Make a note in TestOfAccountConfigurationController about not using
unrelated DAOs in tests