Add linting to CI and pre-commit

[mgax]

I was reviewing #94 and found myself making a number of stylistic suggestions that should have been caught by a linter:

• Newlines at the end of files
• Indentation in templates
• Mixed quotes in templates

We can just pull the linting setup that https://github.com/wagtail/cookiecutter-wagtail-package configures for new packages.

[Stormheg]

Fixed now that #96 is merged