- Center the +/- glyph in the expand control: its line-height (1.25em)
was larger than the 1em box, pushing the glyph down. Derive the box's
margin-top from its size so it is centered in the row (desktop and
the larger mobile variant).
- Hide sort arrows on non-orderable headers. The default initial order
targets column 0, the expand column, which got a sorting_asc arrow
and arrow padding although it cannot be sorted.
- Center row checkboxes level with the expand control by using
Bootstrap's 0.25em margin-top instead of a fixed 7px.
Signed-off-by: Moritz Stückler <moritz@bitbetter.de>