Implement API v2 (#13)

* Add documentation notice

* Rename the deletion token to modification token

* Implement API basics

* Implement paste modification endpoint (#10)

* Implement paste metadata

* Implement report webhook support

* Document API

* Document paste entity

* Update syntax highlighting types (js -> jsonc)

* Update migrator

internal/storage/postgres/migrations/000003_add_metadata.up.sql

@@ -0,0 +1,5 @@
+begin;
+
+alter table if exists "pastes" add column "metadata" jsonb not null;
+
+commit;