Added clear-left and clear-right classes to Grids SCSS and migrated common clear properties to placeholder selector. Addresses issue #1122.
Increased specificity of clear placeholder selector's name in Grids SCSS.
Merge pull request #1151 from EricDunsworth/v3.1-grids-clear-left-right

Added clear-left and clear-right classes to Grids SCSS and migrated comm...