Add sales channels (#1103)

- [x] Data model
- [x] Enforce constraint
- [x] Filter order list
- [x] Set channel on created order
- [x] Products API
- [x] Order API
- [x] Tests
- [x] Filter reports
- [x] Resellers
- [ ] deploy plugins
  - [ ] posbackend
  - [ ] resellers
  - [ ] reports
- [x] Ticketlayouts
- [x] Support in pretixPOS

src/pretix/base/channels.py

@@ -0,0 +1,66 @@
+import logging
+from collections import OrderedDict
+
+from django.dispatch import receiver
+from django.utils.translation import ugettext_lazy as _
+
+from pretix.base.signals import register_sales_channels
+
+logger = logging.getLogger(__name__)
+_ALL_CHANNELS = None
+
+
+class SalesChannel:
+    def __repr__(self):
+        return '<SalesChannel: {}>'.format(self.identifier)
+
+    @property
+    def identifier(self) -> str:
+        """
+        The internal identifier of this sales channel.
+        """
+        raise NotImplementedError()  # NOQA
+
+    @property
+    def verbose_name(self) -> str:
+        """
+        A human-readable name of this sales channel.
+        """
+        raise NotImplementedError()  # NOQA
+
+    @property
+    def icon(self) -> str:
+        """
+        The name of a Font Awesome icon to represent this channel
+        """
+        return "circle"
+
+
+def get_all_sales_channels():
+    global _ALL_CHANNELS
+
+    if _ALL_CHANNELS:
+        return _ALL_CHANNELS
+
+    types = OrderedDict()
+    for recv, ret in register_sales_channels.send(None):
+        if isinstance(ret, (list, tuple)):
+            for r in ret:
+                types[r.identifier] = r
+        else:
+            types[ret.identifier] = ret
+    _ALL_CHANNELS = types
+    return types
+
+
+class WebshopSalesChannel(SalesChannel):
+    identifier = "web"
+    verbose_name = _('Online shop')
+    icon = "globe"
+
+
+@receiver(register_sales_channels, dispatch_uid="base_register_default_sales_channels")
+def base_sales_channels(sender, **kwargs):
+    return (
+        WebshopSalesChannel(),
+    )