Switch mailgun webhooks to the new Mailgun webhook api

[rohankapoorcom]

Description:

Home Assistant currently supports webhooks from the mailgun event api. Mailgun released a new webhook api [reference docs] which is much better (uses JSON!) and provides signed payloads for authentication.

Since we've already merged in a breaking change to dev for mailgun (migrating to the webhook component #17464 (slated for the 0.82.0 release) which requires users to make changes to their mailgun configuration to get events into Home Assistant, we could avoid a second breaking change by switching to the new api now.

Related issue (if applicable): related to #17464

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#7208

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

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:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

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

[houndci-bot]

Some files could not be reviewed due to errors:

Traceback (most recent call last):

Traceback (most recent call last):
  File "/home/linters/.local/bin/flake8", line 7, in 
    from flake8.main.cli import main
ModuleNotFoundError: No module named 'flake8'

[houndci-bot]

Some files could not be reviewed due to errors:

Traceback (most recent call last):

Traceback (most recent call last):
  File "/home/linters/.local/bin/flake8", line 7, in 
    from flake8.main.cli import main
ModuleNotFoundError: No module named 'flake8'

[rohankapoorcom]

This required a change to the strings.json. I didn't see anything in the docs that mentioned how to handle string changes when things are already translated. I ran a find/replace across all of them - let me know if there's a different way I should have handled that.

[houndci-bot]

Some files could not be reviewed due to errors:

Traceback (most recent call last):

Traceback (most recent call last):
  File "/home/linters/.local/bin/flake8", line 7, in 
    from flake8.main.cli import main
ModuleNotFoundError: No module named 'flake8'

[houndci-bot]

Some files could not be reviewed due to errors:

Traceback (most recent call last):

Traceback (most recent call last):
  File "/home/linters/.local/bin/flake8", line 7, in 
    from flake8.main.cli import main
ModuleNotFoundError: No module named 'flake8'

[balloob]

🎉