Understand and document user management

[olivierthereaux]

As we pick up pace on the re-development of Octopub and deal with tech debt, one of the areas worth looking at in greater details is the user management.

From superficial github archaeology, it looks as though user roles were developed in early 2017 but never fully documented. It makes it a little hard to pick up, debug, and further develop.

This issue should be (first) a repository of what we find about this set of features, and (then) help us track the proper documentation of:

• user management features, as implemented -> should go into the wiki/readme
• any known problems with user management -> create new issues
• features we may deprecate -> create new issues, engage with community to sense check
• how we may evolve the features -> ditto above