bots: Allow incoming webhook bots to be configured via /bots.

Without disturbing the flow of the existing code for configuring
embedded bots too much, we now use the config_options feature to
allow incoming webhook type bot to be configured via. the "/bots"
endpoint of the API.

zerver/lib/users.py

@@ -49,7 +49,30 @@ def check_short_name(short_name_raw: str) -> str:
 
 def check_valid_bot_config(bot_type: int, service_name: str,
                            config_data: Dict[str, str]) -> None:
-    if bot_type == UserProfile.EMBEDDED_BOT:
+    if bot_type == UserProfile.INCOMING_WEBHOOK_BOT:
+        from zerver.lib.integrations import WEBHOOK_INTEGRATIONS
+        config_options = None
+        for integration in WEBHOOK_INTEGRATIONS:
+            if integration.name == service_name:
+                # key: validator
+                config_options = {c[1]: c[2] for c in integration.config_options}
+                break
+        if not config_options:
+            raise JsonableError(_("Invalid integration '%s'.") % (service_name,))
+
+        missing_keys = set(config_options.keys()) - set(config_data.keys())
+        if missing_keys:
+            raise JsonableError(_("Missing configuration parameters: %s") % (
+                missing_keys,))
+
+        for key, validator in config_options.items():
+            value = config_data[key]
+            error = validator(key, value)
+            if error:
+                raise JsonableError(_("Invalid {} value {} ({})").format(
+                                    key, value, error))
+
+    elif bot_type == UserProfile.EMBEDDED_BOT:
         try:
             from zerver.lib.bot_lib import get_bot_handler
             bot_handler = get_bot_handler(service_name)