Add automated "compose new" E2E web test for Chrome

[sbrudz]

As we work on adding support for additional browsers, such as Firefox #68, it will become increasingly important to run tests on all of those browsers to ensure that the plug-in works as expected and does not inadvertently break any Gmail functionality.

The goal of this ticket is to add an automated web tests that:

1. logs in to a test Gmail account
2. opens a compose window
3. types text that should result in a warning being shown
4. ensures that the underline is showing up and the correct popup is shown
5. confirms that it is still possible to send this email by sending it to the test Gmail account

With the addition of the web-ext tool in #89, it is now possible to start an instance of Chrome with the plug-in installed.

GitHub Actions can be used to run E2E web tests on various platforms. For example: https://github.com/cypress-io/github-action