[Fix #61] User-editing UI now accessible.

It's not the solution we wanted, but it's the solution we have. At
least until we spin up a WebSockets implementation for 1.0.  Any user
can visit his own profile page and from there, she and *only* she can
edit her profile data including name, email address and user
bio/profile content (as Markdown).

* Added `app/assets/javascripts/users.js.coffee` to make button element
(as opposed to link) work;
* Added `#profile_bio_header` method to `UsersHelper`;
* Added new `ProfileBioHeaderBuilder` class to support the above;
* Replaced the "raw" header element in app/views/users/show.slim with a
call to `#profile_bio_header`;
* New feature specs to verify correct button visibility for
  * guest user;
  * registered user other than profile owner;
  * profile owner.

RSpec: 571 examples, 0 failures
RuboCop: 139 files inspected, no offenses detected