[WiP] Fix trial expiration

Separating plan_level from community to a new table for the sake of saving some data about when communities have changed their plan levels.
No need to have `@current_community` in the if-condition. Now that sharetribe.com marketing site is in separate service, we can be sure that we always have a community, thus:

```
@current_user && @current_user.has_admin_rights_in?(@current_community)
```
...is good enough.