evolution: Find a solution to keep track of users who edited an interview about evolution

[tahini]

Part of the participant/user split

[tahini]

Here's a proposal for this one:

• Add a table to the database, with 4 fields: user_id, interview_id, opened_at, last_update. The last 2 fields are timestamps.
• opened_at would be set upon activation of the interview
• updated_at or last_update would be set at every update, to the most recent entry per user_id/interview_id
• Those update can be done through a request middleware on the activeInterview and updateInterview express routes. That middleware would be optional (defaults to a function that simply passes through), and present only on the admin side.

We could also add a fifth column: for_validation (boolean), which would be true if the interview was opened by a user for validation, and false if for edition. PK would be user_id, interview_id, for_validation

Note The optional middleware mechanism can also be used for logging purpuses (#47), which would decouple the interview from the logic of logging, which are totally unrelated.

[kaligrafy]

We should use updated_at to be consistent with other updated_at fields elsewhere in evolution and transition

[kaligrafy]

maybe replace opened_at by viewed_at