[Fix #3] Standardise view templates on Slim.

Existing Haml and ERB templates removed. Haml Gem removed from Gemfile. Since ERB is soldered into Rails (via `erubis`), actually removing it is more trouble than it's worth — for now.

RSpec: 5 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 9 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2