Allow specifying custom html urls to load.

[andrey-git]

Description:

Allow specifying custom html urls to load.

Proposed release text:

The frontend component now supports loading custom html files when Home Assistant front end starts.
It could be Javascript, CSS or custom Web Components.
Along there was a breaking change in the way Custom UI is used:

• custom_ui_state_card now specifies the exact element name and not a suffix after state-card. So if you had state_card_custom_ui: custom-ui in your config, you should change it to state_card_custom_ui: state-card-custom-ui
• custom_ui_state_card no longer makes the html fetch, you should make it separately:

frontend:
  extra_html_url:
    - /local/custom_ui/state-card-custom-ui.html

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#3260

Example entry for configuration.yaml (if applicable):

frontend:
  extra_html_url:
    - https://example.com/file1.html
    - /icons.html

Checklist:

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

• Documentation added/updated in home-assistant.github.io

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[mention-bot]

@andrey-git, thanks for your PR! By analyzing the history of the files in this pull request, we identified @balloob, @JshWright and @fabaff to be potential reviewers.

[balloob]

Let's just take in 1 url.

I'm actually not too convinced that we need this method just yet. What would be a usecase that we want to programmatically add these?

[andrey-git]

Changed to take 1 url

[andrey-git]

The reason for add_extra_html_url method is to allow custom backend+frontend work.

For example I have a custom component that registers my custom panel and will add this extra url.