Add states and transitions for donations onboarding

[joshsmith]

Problem

We need to rewrite the Transition.UserState module to allow for some flexibility with signing up from the donations page:

• signed_up becomes signed_up_normally
• Move apply_state_transition to the regular changeset
• Add signed_up_donating
• Add donated
• Allow for a state to be set with sign_up_normally, sign_up_donating, and donate
• Can transition from signed_up_donating to donated
• Possibly add a relationship to the project they're donating to when signing up
• Can transition from donated to edited_profile via edit_profile

[joshsmith]

This has been implemented, though not precisely as described above. See #776