Move external view types to common

[colin-grant-work]

What it does

I noticed that there was an import running from the common folder to the plugin folder, which is not desirable - plugin and webview should be able to import from common, but not vice-versa.

How to test

Review checklist

• [x] as an author, I have thoroughly tested my changes and carefully followed the contribution guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the code of conduct