Plex external config flow

[jjlawren]

Description:

Instead of prompting a user to provide an existing token, will redirect them to the plex.tv website to complete authentication and will obtain a token on their behalf with an OAuth-like process. Relies on new library plexauth.

This new auth method has the added benefit of showing a separate "Home Assistant" authorized device on the Plex account, instead of the legacy method of hijacking an existing authenticated device's token.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#10462

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[MartinHjelmare]

Great!