Fix image files presentation at blob view and diff view.
Fix ISSUE schema.

- MILESTONE_ID is NULLABLE
- Add ASIGNED_USER_NAME column
Display parent commit id at the commit details page.
Merge branch 'master' of https://github.com/takezoe/gitbucket.git
Merge branch 'master' of https://github.com/takezoe/gitbucket.git
Enhance repository creation / deletion for issues:

- Insert initial record into ISSUE_ID in repository creation
- Delete all records of ISSUE_ID and ISSUES in repository deletion