Replace deprecated classes with new ones.

This commit squashes together a number of very small commits to make the
history a little more manageable. The details are preserved below for
posterity.

Replace .row-fluid with .row.
Replace .span* with .col-md-*.
Replace .brand with .navbar-brand.
Replace .nav-collapse with .navbar-collapse.
Replace .btn-navbar with .navbar-btn.
Replace `.btn` with `.btn .btn-default`.

 - Bootstrap 3 splits the definition of an item ("button") from the
   theming of the item ("btn-default"). As it turns out, although we use
   `.btn` all over the place for obvious reasons, we only have a couple of
   usages where the button is left with its default theme: all the rest
   override the theme for, e.g., a primary or a danger button.

Replace .control-group with .form-group.
Replace `.checkbox.inline` with `.checkbox-inline`.
Replace .input-prepend with .input-group.
Replace .add-on with .input-group-addon.
Replace .muted with .text-muted.
Replace .bar with .progress-bar.
Remove usages of deleted classes.

This commit squashes together a number of commits to make the history more
manageable. The detail is preserved below for posterity.

Remove .form-search.
Remove .controls.
Remove .navbar-inner.