Fix how to pick/unpick posts of other users via API

[Daniel Zoller]

Currently you can pick posts of other users by calling /users/[otheruser]/clipboard/[resourceHash]. This is a little bit confusing because /users/[otheruser]/clipboard represents the clipboard of the user [otheruser].

Fix this by:

1. do not allow users to access the clipboard of other users
2. new clipboard entries should be POSTed to /users/[user]/clipboard (by providing username and resourceHash)
3. Check if the unpick actions must be fixed as well

[Daniel Zoller]

Please update the rest-client as well!

[Daniel Zoller]

Removing milestone: 2.0.49 (automated comment)