Revert "Merge pull request #2831 from dimagi/revert-vellum"

This reverts commit b3345eae872e9c3c3810ac2f78757a6523a0d6f2, reversing
changes made to 5be1de723d3563157dbe9d6aa4fbc58dca80376e.
Merge branch 'master' into vellum-includes

Conflicts:
	corehq/apps/app_manager/templates/app_manager/form_designer.html
work in progress
remove explicit form_url param and generalize method
add session context endpoint and simplify session access apis
basic support for session resume in cloudcare

- adds a bunch of backbone models and views for sessions
- splits playing the form into two methods so the latter can be shared
- UI could use some work
pass session name (including case name if available) to touchforms
dialog confirm change
whitespace and remove dead comment
basic UI for deleting sessions, move session list to top level
dead code and js syntax fixes
sync session list on form submissions
add app id to sessions
properly set the app in the view when playing a session
add app name to session name
rename previous sessions --> incomplete forms
make styling more general (adds pointer to incomplete forms nav)
fix typo
confirmation for deleting incomplete sessions
update unload text to match form navigation text