Remove unused Activities code

In the past, Crabgrass showed an activities list on the dashboard (or planned doing so).

There is a lot of code dealing with this kind of activities.

created branch 162-remove-unused-activities-code

mentioned in merge request !153 (merged)

@azul: not sure if there were plans to use the activity tracking in the future to show all activities of a certain group.

But as they are not used at the moment, I suggest that we just delete them (and get rid of tons of data).

closed via merge request !153 (merged)

mentioned in commit b4525f74

mentioned in merge request !154 (merged)

mentioned in commit aa089168