Implement full PR review

[aneshodza]

Making a todo list so i dont lose oversight:

• Newlines around headers in readme
• Remove , nil from ENV.fetch
• Move CLIENT into init.rb
• Extract verify_signature into before_action with guard clause
• Use strong params in webhooks controller
• This
• Use Client.compare instead of .commits
• Delete WebhooksHelper
• Extract state in pull_request.rb into a method
• Extract params[:pull_request] in pull_request.rb
• Change multi line string to partial in render
• get_ and set_ in hook_listener
• Use Rails.root.join instead of File.expand_path("#{File.dirname(__FILE__)}
• This
• Use i18n everywhere
• Change time zone to local
• Add an installation guide to the readme
• Extend ENV instead of overwriting

[aneshodza]

Not sure if strong params make sense, as we have the signatures

[aneshodza]

@sihu FYI, this PR is still open. Might make sense to let someone else take this over