Allow managing cloud webhook

[balloob]

Description:

This adds foundation for cloud-extension of webhooks. Allow managing via frontend. Handling incoming webhooks from relayer will be done in another PR.

WIP because I am not done yet with adding tests.

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.