Use `fontello-rails` for Rails-specific code

[paulkoegel]

i think we could do a better job of describing what this gem does, esp. when compared to https://github.com/blackxored/fontello-rails.
main advantage for me is that we offer the possibility to pick desired icons via the web interface and have only those loaded into rails, while the other gem seems to load all of fontello.

[jhilden]

Makes sense.

Maybe we could also just move the rails specific code (https://github.com/railslove/fontello_rails_converter/blob/master/lib/fontello_rails_converter/railtie.rb) to the fontello-rails gem, and either add that gem as an dependency or as an suggestion to people that are using fontello with rails.

@blackxored what do you think?

[blackxored]

@jhilden 👍