Type Activity/Stage/Round/Pool

[jchaffraix]

Those 4 are closely coupled so it made sense to handle them as a single PR.

Unfortunately due to the coupling, the return of some key getters were not added, instead I added a comment explaning the situation. Properly typing will likely require some refactoring to split up the per-type lists, which is better done in a follow-up.

TEST=Checked a user's available rounds, updated a project's state to confirm that the rounds/pools were still listed.