OR-737 Add the core for searching users not in specific community

* Augment `getUsersList` and `getUsersSearch`
* Add storage tests
* Add a helper for adding users in communities
OR-737 Add the documentation
OR-737 Add a route to search users who can be invited in community

* Add the route `GET /api/communities/:id/invitablepeople`.
* Add the webserver controller
* Add midway tests
OR-737 Fix unit test
OR-737 Update domain controller
Merge pull request #531 in OR/rse from ~RPIGNOLET/rse:OR-737 to master

* commit 'c08ae0c0893636417b06df80d7b5f7027999602a':
  OR-737 Update domain controller
  OR-737 Fix unit test
  OR-737 Add a route to search users who can be invited in community
  OR-737 Add the documentation
  OR-737 Add the core for searching users not in specific community