* Switch mailgun to use webhook api
* Generalize webhook_config_entry_flow
* Add tests for webhook_config_entry_flow
* Add tests for mailgun
* Remove old mailgun file from .coveragerc
* Refactor WebhookFlowHandler into config_entry_flow
* Remove test of helper func from IFTTT
* Lint
* Mailgun notify service
* Update dependency to version 1.3
- The provided credentials (including the domain) are now checked during
startup, as requested by @balloob
- The domain name is now optional
- There's a new config item "sandbox" which indicates whether to use the
sandboxed domain in case the domain is not set
* Fix a few lint issues
* Disable lint check no-value-for-parameter