Frontend tweaks for GiveWP form updates

[benlk]

Changes

• Starts separate LESS files for widgets and GiveWP-specific styles
• Sets padding on GiveWP buttons from 12px 24px to 12px
• Makes the Give submit button match the .btn-primary styles
• Fixes <select> height when padded by Give's styles
• Gives widgets with the .rev "Reverse background" class a blue border
• Columns block margins as described in https://github.com/INN/umbrella-inndev/issues/159

Why

For https://github.com/INN/umbrella-inndev/issues/146 and https://github.com/INN/umbrella-inndev/issues/159

Questions

• [ ] What other changes are necessary for the reverse widget styles?

[benlk]

Revisions to widget reverse styles to match colors of non-reverse widget but with border:

On a wider monitor:

Just before we go one-column for mobile:

On large mobile devices:

Further questions:

• The right-hand side of Normal widgets lacks padding; the right-hand side of Reverse widgets has padding. Do we want to add padding to the right-hand side of Normal widgets to match?
• The 8px border and 24px padding on Reverse add to 32px, which doesn't align with the 24px left padding on Normal. Do we want to adjust these to match, and if so, should Normal be padded out or Reverse be thinned?

[benlk]

Approved, according to emails from Sharene and Larry.