Add Retry on http requests (dont cancel migration if one error ocure)

[6543]

gitea Version: 1.11.0+dev-110-gc4bc5abda

Migrating gitea WITHOUT pullrequests and releses works:
https://gitea.com/test-organisation/gitea-mig-test

PullRequest test: https://gitea.com/6543/mig-test2
Releases test: https://gitea.com/6543/rel

[lunny]

Once there is any network error on the migration, it will rollback. So we have to add some retry for all the http request.

[6543]

@lunny title changed ...