Add Migration from gogs
Problem
As I decided to stop to maintain gogs and to help gitea for the maintenance we need to give a possibility to the user to migrate to gitea without losing some data.
Solution
A a migration to be able to migrate from gogs to gitea and to keep all data.
PR status
Tested on some VM and with 2 installation. Should work, but we can't be sure that no issue will happens. Might need some other tests with user with good knowledges.