Refector session view list to allow views from multiple databases

Changes include:
* Expect all "views" RefList items in a Session to have a "db" field,
pointing to its ImageStore
* Allow a session to contain views from multiple ImageStores
* Session is no longer reqired to contain an "image_store" field
* Move code to fetch and generate Session details into a single location
in APIv2
* Fix mismatched HTML tags in the session template
* Rename the Session model field to "hide_annotations"
* Rename the View model field to "hidden_label"
Remove "name" field from Session

This field didn't have any function and was often set incorrectly.
Merge branch 'session-cleanup'

* session-cleanup:
  Remove "name" field from Session
  Refector session view list to allow views from multiple databases
  Clean whitespace
  Remove dead code for editing a view label inline
  Improve the default JSONEncoder to be more robust