[Fix #69] View template/helper cleanup.

Three lines of markup and helper calls in `app/views/users/show.slim`
replaced by a single-line call to a single helper. Improvement; the
previous template code conveyed irrelevant detail.

To make *that* work, the `ProfileBioRowBuilder#to_html` method was
fundamentally redone. No longer directly using Rails `content_tag` and
`concat` helper methods to sling HTML together, it now uses Nokogiri; a
much more predictable way to do the job.

All this also meant that the `UsersHelper` methods
`#profile_bio_header` and `#profile_bio_panel` are no longer needed by
view code, so they've been eliminated. Much better, yes?

RSpec: 762 examples, 0 failures
RuboCop: 174 files inspected, no offenses detected