The use of the `rgba` function, which was added to work around a bug in
Safari, prevents users from setting `$rgba-text-colour` to inherit.
This is useful if you wish to control the text colour by cascading a
style down from, say, a body tag, which in turn is useful in order to
apply this control at run-time rather than build-time.

This fixes the problem by first testing for the value `inherit`. When
found we simply evaluate to `inherit`, which is probably the best we
can do in the circumstance. When it is not found we apply the `rgba`
trick.

Adds some tests for the reinstated support for `inherit` as a
`$govuk-text-colour` in the `govuk-link-style-text` mixin.

We wish to defend against passing `rgba()` a value that is not a colour, such as is the case with `inherit`.

Rather than testing for `inherit` specifically it is better to just test the type of the variable to see whether it is a colour or not.

We can also avoid setting the `:hover` state entirely which should reduce the size of the resulting CSS.

(Thanks to @36degrees for this suggestion.)

Co-authored-by: Oliver Byford <[email protected]>

The last commit optimised the code to remove the `:hover` state when it
isn't required, therefore we must no longer test for its presence.