Issue #13 and issue #61 p1

Implemented Activity detail Edit/creation
90%, "other links" needs to be implemented, but I think doing a custom element to manage a list of links can be reused for the Original Post selection too.

The code works, but it got a bit messy and would be a better idea to create a new controller to manage, at least the singleActivity Edit

OT. Sorry I'm late.. got some fighting to do with the tabs and core-pages selection
Not for this PR, but as a future enhancement: Should we represent Activity Groups with this data structure in the API/DataStore as well?