JetBrains: Make auth more convenient

Created by: vdavid

Use the oauth-like solution that @abeatrix developed for VS Code.

@philipp-spiess's clues from the GitHub plugin:

They have a regular login button, and also this UI:

They implement the oauth callback by using a custom webserver they start: http://127.0.0.1:63343/api/github/oauth (but the port is probably dynamic) – source code

More granular task list:

• Add "User authentication" and "Navigation settings" groups
• Set up URL syntax validation
• Set up automatic access token validation after an access token change or URL change
• Add help texts
• ...more coming...