Add a sanitize_value implementation for the color input

I had to change the test a little bit to avoid some failures due to
color and text both having a sanitizedValue which was making the test
use the first assertion instead of the second one in some cases.

The sanitize_value implementation is pretty simple, we iterate over the
content and checks that the content is 7 characters long, that the first
character is a `#` and then that all the following characters are
hexadecimal. If all those requirements are met, we lowercase the
content, otherwise we put `#000000` in it.

components/atoms/static_atoms.txt

@@ -1,3 +1,4 @@
+DOMContentLoaded
 abort
 activate
 beforeunload
@@ -10,13 +11,13 @@ characteristicvaluechanged
 checkbox
 click
 close
+color
 controllerchange
 cursive
 date
 datetime
 datetime-local
 dir
-DOMContentLoaded
 email
 emptied
 error