Migrate backup/restore from Gogs

[bkcsoft]

gogs/gogs@ca2cfaf

PR #1637

[bkcsoft]

@tboerger @lunny @andreynering @metalmatze Anything I've missed in the todo-list?

[pgaskin]

Add to features list: https://github.com/go-gitea/docs/blob/master/content/page/index.en-us.md

[lunny]

Add deprecation notice to gogs dump maybe you mean gitea dump?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.