Check-In crowdactions

[rubenhorn]

Some crowdactions should require/offer periodic check-in by participants to continuously track/confirm their participation.
This requires some changes to the API, backend logic and internal state of the participations.

Changes:

• Track "streak" of participation
• Allow check-in to "extend streak"
• Reset streak if check in was missed (should the maximum streak still be stored?)
• Update API docs

[rubenhorn]

@tomsiebring @Xazin This feature might need to be defined a bit more clearly, before we can start to implement it.