Maintain stable column width when showing/hiding sort indicators

Prior to this commit, sorting a table column would cause the column
sizes to change because it would add and/or remove pseudo elements upon
showing or hiding the triangle indicators.

This commit makes the column width stable by including the indicator at
all times in all columns, and using `visibility: hidden/visible` to
toggle its, er, visibility.

Change-Id: Iddacc146a5a4906d32e24301a22a8e370e3150f2
Reviewed-on: https://gerrit.causes.com/26994
Tested-by: Greg Hurrell <greg@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>