Use black instead of gray as input font color in Bootstrap forms

The gray color made it look as if the inputs were not editable.

Test Plan:
- Grep for "bootstrap/less/forms.less" and confirm that the only place we're
  using it is in "settings.less"
- Load /settings and confirm that the inputs have black text instead of gray
  (placeholder text is still gray though, as desired)

Auditors: benkomalo