Move client authentication into its own middleware and provide
differentiation between HTTP requests that do not provide
credentials (401) and requests that authenticate as a non-admin
user (403).
Closes#152
This commit moves the user.Manage to its own package (user/manager)
so it can import the connector package in a later commit.
For clarity, it renames "Manager" to "UserManager" using gorname.
This commit has no functional changes.
Invite emails are essentially just reset password emails with a
different template (though this can and probably will change (slightly)
in the near future)
