Get info about organisations from database table

This is the first step of replacing the `domains.yml` file.

In order to replicate the same functionality we get from the
`domains.yml` file and its associated code this commit adds a
`Organisation` model. This model copies a lot of methods from the
`AgreementInfo` class which wrapped the `domains.yml` file.

It factors out some stuff that would otherwise be duplicated between the
`Organisation` and `Service` model, in such a way that could be reused
for making other models in the future.

This commit doesn’t change other parts of the code to make use of this
new model yet – that will come in subsequent commits.

app/models/__init__.py

@@ -0,0 +1,27 @@
+from flask import abort
+
+
+class JSONModel():
+
+    ALLOWED_PROPERTIES = set()
+
+    def __init__(self, _dict):
+        # in the case of a bad request _dict may be `None`
+        self._dict = _dict or {}
+
+    def __bool__(self):
+        return self._dict != {}
+
+    def __getattr__(self, attr):
+        if attr in self.ALLOWED_PROPERTIES:
+            return self._dict[attr]
+        raise AttributeError('`{}` is not a {} attribute'.format(
+            attr,
+            self.__class__.__name__.lower(),
+        ))
+
+    def _get_by_id(self, things, id):
+        try:
+            return next(thing for thing in things if thing['id'] == str(id))
+        except StopIteration:
+            abort(404)