Add `user_may_edit` boolean to event

[Rikuoja]

For authenticated users, the API knows whether the user has rights to edit each event or not (already used for displaying fields such as created_by and last_modified_by, https://github.com/City-of-Helsinki/linkedevents/blob/master/events/api.py#L452).

Adding this field would be as easy as adding a field user_may_edit=True to only_admin_visible_fields.

[Rikuoja]

Required for City-of-Helsinki/linkedevents-ui#532

[Rikuoja]

Tho, there is a catch. The user may not edit all events that display those fields, if they are in different branches of the same organization tree :D Creator data is shown for admins within the organization even if they may not edit the event.

So, another check is indeed needed to actually see whether we should return True or False.

[Rikuoja]

And, indeed, we should probably display this as True for regular users in draft events, even though they do not see the other admin fields at the moment :D