Apply defra-ruby-style and fix issues

[Cruikshanks]

Currently, we are not linting the TCM within our CI.

This is for 2 reasons

• we get a large number of violations reported from simply running vanilla rubocop against the project which suggests it was not common practice to do so
• Defra's standard for Ruby is to use defra-ruby-style so there seems little point in fixing any existing violations until this is applied

So to get the project up to Defra standard and allow is to start linting changes automatically we first need to implement defra-ruby-style, then fix any issues it highlights.

[Cruikshanks]

Fixed by https://github.com/DEFRA/sroc-tcm-admin/pull/327

We still have some issues to do with things like class and method size recorded in a .ruboco_todo.yml. But almost 5,000 issues and almost a 100 separate cop violations have been resolved.