Move helper method to common location for re-use.

Change-Id: I64d3d73ae098652d115fb130f99007857c54ebcb
Make it possible to remove a registration entry.

Change-Id: I565733b3042e6f1a33cd90586ab5e9edf66f907b
Implement the account activation endpoint.

This provides most of the desired implementation plus tests. Users
can now click on the links sent to them whent the team-leader registered
their accounts & get to a page which makes the account usable & tells
them their new password (and to go change it).

Things it's missing:
* expiration of the activation links,
* notifications back to the teacher (either for expiration or activation)
* the wording in the emails and success page need review

Change-Id: I15f5a6c578d43f550ca8308672698bd1107ce3c2