AUS 3898: Establish Linting with ESLint

[SamuelBradley]

This is a change to establish basic linting as part of this project, to better enable collaboration and to catch more potential issues.

• Old default TSLint has been removed
• New ESLint has been added using this guide as a starting point https://medium.com/acute-angular/migrating-angular-projects-from-tslint-to-eslint-and-prettier-cd87eea9ffcd
• Fixed a number of "low hanging fruit" linting errors
• Changed the status of more numerous/complex linting errors to warnings
• Created a github action to run linter on all AUS branches and PR's
• No formatting rules are currently included as part of this PR to avoid too much change