UI improvements: Color & contrast

[jeremystretch]

Proposed Changes

This issue addresses some items from our recent UI evaluation, specifically those related to color and contrast.

• Adopt original BS3 color palette for primary/success/warning/etc. classes
• Fix login banner color (too light in light mode)
• Use a lighter color for top-level navigation menu items
• Home view panel headers should not look like links
• Don't style table column headers as links
• Use a more subtle color for footer link icons
• Add & import buttons in nav menu should stand out less
• Use a single color/style for all top-right object buttons (add, import, export, etc.)
• Dropdown widget caret should be lighter color
• Don't color forms fields green on submit
• Logout link in user menu should not be red
• Top search box should be visually distinct from page body (or not sticky?)

Justification

Cosmetic improvements to the user interface

[heroin-moose]

Table header colors were great though.NetBox looks much more alive with colorful headers.