Replace `Repo.get(Module, id)` with `Module.get_by_id(id)` where possible
This MR:
- Replaces
Repo.get(Activity, id)
=>Activity.get_by_id(id)
- Replaces
Repo.get(User, id)
=>User.get_by_id(id)
- Does not add
Activity.get_by_id/1
andUser.get_by_id/1
, they exist for quite awhile.