MM-49540: drafts migration adding metadata (#7240)

* MM-49540: drafts migration adding metadata

Adds a drafts table migration adding the column "metadata".
This column is going to be primarily used to store the draft's
metadata priority field.

* Addresses review comments

types/database/models/servers/draft.ts

@@ -25,6 +25,8 @@ declare class DraftModel extends Model {
 
     /** files : The files field will hold an array of files object that have not yet been uploaded and persisted within the FILE table */
     files: FileInfo[];
+
+    metadata?: PostMetadata;
 }
 
 export default DraftModel;