Placeholder text colour contrast doesn't meet the accessibility standards

Created by: AlicjaSuska

Currently used placeholder text color (#2B3750 50% opacity) doesn't meet the accessibility standards. My proposition is to change it to #6C7584 100% #2b3750 70% opacity. This is the closest color to the currently used one, that meets the standards.

Placeholder text is used in many places:

• main search bar ghost text 'Search code'
• query builder text fields
• site admin - user stats search bar
• text fields in profile settings etc.

See the document that showcases current color and new proposition with contrast ratio test results.

@rrhyne @christinaforney @felixfbecker Please, review. Thank you!