Add a facility to allow user's username to be renamed

[adamretter]

This is relatively complex as eXist-db/Elemental/FusionDB currently stores users by username rather than id, this will require some upstream fixes first:

• [feature] Add a facility to allow user's username to be renamed eXist-db/exist#4026
• Add an API facility to allow user's username to be renamed fusion-studio-api#71