add 1_webhook_avatars migration

manage/cmd/migration/scripts/1_webhook_avatars.sql

@@ -0,0 +1,13 @@
+-- webhook_avatars table. check issue 46.
+CREATE TABLE IF NOT EXISTS webhook_avatars (
+    webhook_id bigint,
+
+    -- this is e.g a sha256 hash of EmbedURL.to_md_url
+    hash text,
+
+    -- we don't hardcode the mediaproxy url here for obvious reasons.
+    -- the output of EmbedURL.to_md_url goes here.
+    md_url_redir text,
+
+    PRIMARY KEY (webhook_id, hash)
+);