Move raw API and event schemas into /data directory

Historical note: this was originally a series of several commits, spread out
over several weeks. They have been squashed together to make `git annotate`
work properly.

The original commits were:
 * 91ab3934 <Will> 2021-01-25 21:16:42 -0800 Add raw API end event schemas into /data directory
 * aae22f47 <Will> 2021-01-25 21:33:06 -0800 Remove non-data files
 * 1092d4ca <Will> 2021-01-26 20:41:33 -0800 Add data-compatiuble extension (.yaml) to all data files that currently omit one
 * 21060109 <Will> 2021-01-26 20:57:28 -0800 Remove symlink to event-schemas, and update openAPI schema paths accordingly
 * 4f633845 <Travis Ralston> 2021-04-12 21:54:54 -0600 Fix event schema examples too
 * 301c7b2f <Will> 2021-02-05 10:15:42 -0800 Restore docs describing OpenAPI extensions that we use

data/event-schemas/schema/m.identity_server.yaml

@@ -0,0 +1,23 @@
+---
+allOf:
+  - $ref: core-event-schema/event.yaml
+description: |-
+  Persists the user's preferred identity server, or preference to not use
+  an identity server at all, in the user's account data.
+properties:
+  content:
+    type: object
+    properties:
+      base_url:
+        type: string
+        description: |-
+          The URL of the identity server the user prefers to use, or ``null``
+          if the user does not want to use an identity server. This value is
+          similar in structure to the ``base_url`` for identity servers in the
+          ``.well-known/matrix/client`` schema.
+  type:
+    enum:
+      - m.identity_server
+    type: string
+title: Identity Server Preference
+type: object