Reuse custom entered variables for manual jobs when retrying

Problem

With https://gitlab.com/gitlab-org/gitlab-ce/issues/24935 entering variables for manual jobs will become possible. However if such a job fails a user will need to manually reenter every variable previously inputted.

Further information

Coming from https://gitlab.com/gitlab-org/gitlab-ce/issues/24935#note_187875219

Proposal

Allow retrying of manual job with custom entered variables if retried by the same user.