[Web] Align the table expand control and row checkbox

- 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>

data/web/css/build/015-responsive.css

@@ -191,7 +191,7 @@
       height: 2rem;
       width: 2rem;
       line-height: 2rem;
-      margin-top: -15px;
+      margin-top: calc(-1rem - 1.5px);
   }
 
   li .dtr-data {