Make notes and draft notes positions queriable

Problem

The table notes has columns original_position, position and change_position with type text, same happens with the newest draft_notes. In DB level it persist a "Ruby object" (YAML I assume?) string:

gitlabhq_development=# SELECT position FROM "draft_notes" limit 1;
                        position
--------------------------------------------------------
 --- !ruby/object:Gitlab::Diff::Position               +
 attributes:                                           +
   :base_sha: 1c59dfa64afbea8c721bb09a06a9d326c952ea19 +
   :start_sha: 1c59dfa64afbea8c721bb09a06a9d326c952ea19+
   :head_sha: af14638a616cd93a0757e35b1fc69c3611065b72 +
   :old_path: ".gitlab-ci.yml"                         +
   :new_path: ".gitlab-ci.yml"                         +
   :position_type: text                                +
   :old_line:                                          +
   :new_line: 27                                       +

(1 row)

In general that makes it impossible to query with postgres -> syntax as it's not a json or jsonb column. It limits optimizations where we'd like to query for specific file paths and should keep biting us in other places.

We're not able to answer (with queries) how many comments are there in the latest diff version, or how many draft notes. Everything needs to be made in memory now.

Proposals

Either converting all these records to jsonb, or create a diff_positions table to store this data. notes and draft_notes could instead refer to position_id, original_position_id and change_position_id.