Migrate Mailgun to use the webhook component (#17464)

* 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

homeassistant/helpers/config_entry_flow.py

@@ -1,7 +1,10 @@
 """Helpers for data entry flows for config entries."""
 from functools import partial
+from ipaddress import ip_address
+from urllib.parse import urlparse
 
 from homeassistant import config_entries
+from homeassistant.util.network import is_local
 
 
 def register_discovery_flow(domain, title, discovery_function,
@@ -12,6 +15,14 @@ def register_discovery_flow(domain, title, discovery_function,
                 connection_class))
 
 
+def register_webhook_flow(domain, title, description_placeholder,
+                          allow_multiple=False):
+    """Register flow for webhook integrations."""
+    config_entries.HANDLERS.register(domain)(
+        partial(WebhookFlowHandler, domain, title, description_placeholder,
+                allow_multiple))
+
+
 class DiscoveryFlowHandler(config_entries.ConfigFlow):
     """Handle a discovery config flow."""
 
@@ -84,3 +95,50 @@ class DiscoveryFlowHandler(config_entries.ConfigFlow):
             title=self._title,
             data={},
         )
+
+
+class WebhookFlowHandler(config_entries.ConfigFlow):
+    """Handle a webhook config flow."""
+
+    VERSION = 1
+
+    def __init__(self, domain, title, description_placeholder,
+                 allow_multiple):
+        """Initialize the discovery config flow."""
+        self._domain = domain
+        self._title = title
+        self._description_placeholder = description_placeholder
+        self._allow_multiple = allow_multiple
+
+    async def async_step_user(self, user_input=None):
+        """Handle a user initiated set up flow to create a webhook."""
+        if not self._allow_multiple and self._async_current_entries():
+            return self.async_abort(reason='one_instance_allowed')
+
+        try:
+            url_parts = urlparse(self.hass.config.api.base_url)
+
+            if is_local(ip_address(url_parts.hostname)):
+                return self.async_abort(reason='not_internet_accessible')
+        except ValueError:
+            # If it's not an IP address, it's very likely publicly accessible
+            pass
+
+        if user_input is None:
+            return self.async_show_form(
+                step_id='user',
+            )
+
+        webhook_id = self.hass.components.webhook.async_generate_id()
+        webhook_url = \
+            self.hass.components.webhook.async_generate_url(webhook_id)
+
+        self._description_placeholder['webhook_url'] = webhook_url
+
+        return self.async_create_entry(
+            title=self._title,
+            data={
+                'webhook_id': webhook_id
+            },
+            description_placeholders=self._description_placeholder
+        )