Admin can make users sponsors from admin interface

Within the Artifact view in activeadmin interface, an admin can "confirm
sponsorship", essentially indicating that a user has paid for the
digitization of an artifact. The sponsorship confirmation can also be
undone/uncomfirmed. Add tests for the Artifact admin interface.
Configure database_cleaner to actually clear out the db for tests.
Removes 'cross_reference' solr field from ingest process and CatalogController config -- effectively removes that field from search resuts and display.
Sponsorships removed from user on request withdraw

Uncovered this weird bug that blocked me for part of the day: upon
request_withdraw on an artifact, the sponsorship/user was removed from
the artifact itself, but it wasn't being removed from the user, which
was causing an issue at the view layer (on dashboard) in certain edge
cases. Now we are reloading the relationship immediately upon withdraw
which resolves the issue I was experiencing.
Dashboard improvements, shows sponsorship status
Merge pull request #116 from WGBH/rm-cross-reference-solr-field

Removes 'cross_reference' solr field from ingest process and CatalogCont...
sidebar for user dash and layout
Admin can change state to digitizing

In active_admin interface, when viewing an artifact, an admin can change
the state to 'digitizing', which calls the approve_digitization method,
sends out an email update to any users who have requested the item. The
state change is reflected on users' dashboard (who have requested the
artifact).
WIP tests for digitize link activeadmin
Fixes #119 Use single user model instead of two
move user/show spec into feature
Fixes #94 Admin can view artifacts and sort

Admin can sort artifact view by User, state, updated, and
solr_document_id
#79 title of artifact shows in user dash
remove debug line
fixes #84 user page shows sponsored artifacts
 User can track an item already being digitized

If an item is already being digitized courtesy of another user, the
current user can choose to 'track' the item and receive all the same
notifications/email updates and status changes of the artifact on their
user dashboard.
Remove unnecessary user id in dashboard path
User dashboard tests and unfollow feature

Users cannot unfollow an artifact if they are a confirmed sponsor. Users
see a 'following' status on artifacts they've requested. Lots of tests
around the dashboard view.
get travis to rebuild db
modify travis yml
another travis attempt