CssID constructed with Text literals

instead of String literals.

See also

https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/guide-to-ghc-extensions/basic-syntax-extensions

Although in Fay it seems to be better to have modules that have either
String literals or Text literals, not both.

For the server side, we also needed to import Data.String to get the
fromString method and the IsString typeclass.