[Feature] Block editing/removing contributions during lock window

[sthewissen]

Every year between April 1st and July 1st, the existing activities get locked. The app should also do this, just to be on par. However, we can do this client-side or do a server-side check. Both aren't completely foolproof as the user can edit the time on the device or simply not be online. However, the question is whether or not such lengths must be gone to.

• Block editing / removing between April 1st and July 1st.