Added activity page

[Frewacom]

Adds a fully functional activity page that allows you to open, close, and update the activity of every available location.

Some things are still missing, e.g:

• Handle case where the location has disabled activity (Add error message on activity page if a location has activity disabled #125)
• Error handling for API requests (see Add a component to easily create and show errors as dialogs or notifications #126)

I think fixing the above issues should be done in separate PR's, so that we can keep them manageable.

[McFrappe]

Looks great!