refactor: The update checker has become the announcements checker

Replaces June's endpoint with a continuwuity endpoint.
Adds a JSON schema.

Closes #89
Closes #760

docs/static/_headers

@@ -1,3 +1,6 @@
 /.well-known/matrix/*
   Access-Control-Allow-Origin: *
   Content-Type: application/json
+/.well-known/continuwuity/*
+  Access-Control-Allow-Origin: *
+  Content-Type: application/json

docs/static/announcements.json

@@ -0,0 +1,9 @@
+{
+    "$schema": "https://continuwuity.org/schema/announcements.schema.json",
+    "announcements": [
+        {
+            "id": 1,
+            "message": "Welcome to Continuwuity! Important announcements about the project will appear here."
+        }
+    ]
+}

docs/static/announcements.schema.json

@@ -0,0 +1,31 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "$id": "https://continwuity.org/schema/announcements.schema.json",
+    "type": "object",
+    "properties": {
+      "updates": {
+        "type": "array",
+        "items": {
+          "type": "object",
+          "properties": {
+            "id": {
+              "type": "integer"
+            },
+            "message": {
+              "type": "string"
+            },
+            "date": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "id",
+            "message"
+          ]
+        }
+      }
+    },
+    "required": [
+      "updates"
+    ]
+  }