Fix ESLint issues and ensure conformance across code

[lukehesluke]

packages/openactive-integration-tests/.eslintignore contains a list of files which are ignored by ESLint presently. These are old files which use inconsistent conventions and some shorthand styles which are often considered to be ill-advised (e.g. https://eslint.org/docs/latest/rules/no-return-assign)

The goal of this issue is to:

1. Remove that .eslintignore file and fix all ESLint errors that come up.
2. Remove all the eslint-disable comments. These turn off an ESLint rule for an entire file, which is considered to be too broad. When there is a good reason to have a singular exception, comments that simply disable a rule for one line (e.g. eslint-disable-next-line) can be used.
   a. And fix all ESLint errors that come up.