Timeshift table rendering tweaks

- Making the “+/-“ indicators more visually obvious for accessiblity.
Supplied by CSS so they can be independently styled and are slightly
separated from the numbers.
- Adding in “+” indicators
- double math is fuzzy. wheee. Let’s do those margins better
Merge pull request #227 from urbanairship/timeshift-table-visibility-tweak

Timeshift table rendering tweaks