db: add indices where queries takes more than 100 ms

CREATE INDEX cc_pull_request_id_idx ON changeset_comments USING btree (pull_request_id);
CREATE INDEX cs_changeset_comment_id_idx ON changeset_statuses USING btree (changeset_comment_id);
CREATE INDEX cs_pull_request_id_idx ON changeset_statuses USING btree (pull_request_id);
CREATE INDEX pr_org_repo_id_idx ON pull_requests USING btree (org_repo_id);
CREATE INDEX pr_other_repo_id_idx ON pull_requests USING btree (other_repo_id);