Support for ERB Templates

[AliSoftware]

ERB is a great and powerful templating language for Ruby.

By the time we decided to migrate gyro so it uses templates, we didn't know about ERB, but if we knew back in the day I think we'd have chosen ERB over Liquid (the templating engine gyro currently supports/uses).

But ERB would very probably allow us to have way more readable templates than Liquid, better handling of extra spaces/CRLF generated, and are more flexible, so that should probably be the step forward.

Note: Once this is done, we could still support Liquid as a templating engine in the future, since we already did all the work, and thus support both Liquid and ERB, but I think the built-in templates should then still favour ERB for readability and clarity