Allow renaming external users

Supposing that a user gets created in an LDAP system with an incorrect name, gets synced in Bamboo, and only then gets corrected in LDAP, that user's incorrect name is stuck in Bamboo and cannot be corrected through the administration panel.

The current workaround for that is to run this SQL query directly at database level:

update external_entities set name = '<new user name>' where name = '<old user name>';

But that is a really gnarly workaround.