Allow HexColor to take colors with opacity

[zerocrates]

Browsers support, in addition to the 3 and 6 digit color forms we use now, 4 and 8 digit versions that also set opacity at the same time.

Supporting this would be a simple case of updating helper text and some regexes... though we might want to check if any themes, etc. are doing processing in PHP code of the colors they take as settings, as that could run into problems if the colors are suddenly not in the expected format.