Show the username instead of first name in the top navbar.

This mirrors the usability change we made for RBCommons a little while
ago. People know their first name, but sometimes forget the assigned
username. Instead of showing the first name in the top navbar next to
the gravatar, we now show the username instead. This particularly helps
if a first name is not set.
Move column initializers into __init__ calls.

Several of the columns directly assigned member variables that ought to have
been included in __init__ instead. While this was perhaps slightly more
readable, it meant that the Column.__init__ method couldn't operate on them.

This change moves all those parameters into the __init__ calls. It also splits
the "To Me" column into using detailed_label_html and detailed_label, to
dovetail with my djblets change that fixes the sort order in the column
selector.

Testing done:
Looked at the column selector and saw that everything was in alphabetical
order, even when the column included HTML or other entities at the beginning of
the text.

Fixes bug 3260.

Reviewed at https://reviews.reviewboard.org/r/5538/