server/image_search: implement reverse search functionality in postgres

This will remove the dependency on the Elasticsearch database.

The search query is passed currently as raw SQL. Proper implementation
using SQLAlchemy will need custom ORM classed to be made.

Additional config parameter "allow_broken_uploads" has been added.

server/config.yaml.dist

@@ -21,11 +21,15 @@ thumbnails:
     post_width: 300
     post_height: 300
 
+# automatically convert animated GIF uploads to video formats
 convert:
    gif:
      to_webm: false
      to_mp4: false
 
+# allow posts to be uploaded even if some image processing errors occur
+allow_broken_uploads: false
+
 # used to send password reset e-mails
 smtp:
     host: # example: localhost