Placeholder is visually indistinguishable from normal text

Created by: vanillajonathan

The placeholder is almost visually indistinguishable from normal text input. This is confusing and dangerous. The placeholder text ought to be brighter/dimmer.

This was fine in v4.0.0-alpha.5 but is a regression in v4.0.0-alpha.6. The color is defined by the $input-color-placeholder variable in /scss/_variables.scss. From alpha5 to alpha6 it was changed from #999 to $gray-light.