New CurrentUserIdentity class

…which implements/manages concept of "current user". No more do code
and specs arbitrarily write to `session[:user_id]` to define the
logged-in user. Nor does the UserHelper support class
`ProfileBioHeaderBuilder` directly access `sessions[user_id]` to
*query* the logged-in user. In fact, code and specs (outside the
`CurrentUserIdentity` class itself) no longer deal with user *IDs* as
such anymore, but set and query *user* objects. This leads to a
significant amount of cleaning up.

RSpec: 915 examples, 0 failures
RuboCop: 231 files inspected, no offenses detected