Swap the colour of Alert and Attention Modules

I simply swapped the values of the variables $alert
and $attention since color-attention is driven by the
$attention variable we don't have to worry about changing
anything else
Change the color of .color-attention to $alert
Fixed naming of variables

- Added $red and $yellow to _mixins.scss
- $alert and $attention are now driven by new variables
- Changed .color-attention to be created from $red
Merge pull request #1096 from nschonni/backport-1084-to-v3

Backport module colour fixes from #1084 to v3.0