Switch to slim

* Converted all erb templates (except email) to slim via Haml (2nd option at https://github.com/slim-template/slim/wiki/Template-Converters-ERB-to-SLIM)
* Factored out some partials into `shared/_active` and `shared/_errors`
* Fixed a few other inconsistencies in the views
* Tidied up and sorted `Gemfile`