indico: add ability to propose, review and accept tracks

[rppt]

Currently, Indico treats all the submissions like abstracts for talks and it does not work well with MC proposals.
The sessions and tracks need to be created manually for every accepted MC proposal.
It would be very useful to automate this entire process or at least parts of it.

[rppt]

98d1ae6

[rppt]

Commit acb7fbf automates session and track creation but for some reason with these changes the "track" selection does no disappear when you judge as "reject" or "merge" and the form validation fails

[rppt]

added an issue upstream:
indico/indico#6963