Combine duplicate gradients

There are still some outstanding dupes between differend sheets that are
outstanding
Move IE8 button disabled fix to mixin

Modern browser CSS will keep it as a single rule, IE8 CSS duplicates the
rule for the :disabled bug
Remove text-shadow mixin calls

After further research, this mixin doesn't add any value
http://compass-style.org/reference/compass/css3/text-shadow/
- Removed the call in footer-ie since the property is only supported
IE10+
- Since the property is only supported in IE9, a custom mixin may be
desired to polyfill with the MS filter version
- Standardize all calls to use the colour last format
Merge pull request #1199 from nschonni/cleanup-dup-gradients

Combine duplicate gradients
Merge pull request #1200 from nschonni/mixin-ie8-disabled-buttons

Move IE8 button disabled fix to mixin
Maually merged in pull request #1201.
Resolved merge conflicts.