Convert shopping-list clear to WebSockets

[iantrich]

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.

If the code does not interact with devices:

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

[iantrich]

@balloob
I'm planning to add the ability to create multiple lists but also was thinking that it would be good to start storing the shopping-list data in the database rather than in a json file. So a couple questions for you:

1. Is storing to the database rather than a json file worth the trouble?
2. Which do you think should be done first? (Would still support the current shopping-list panel by having a default list Inbox that would be what is used by the panel.

Thanks.

[balloob]

Let's not store anything in the database until we have a clear consensus about component data storage. We can't just have 1 component go rogue.