feature, bulk importing of repositories

[bradder555]

Recently i hacked about in the back-end and database to migrate all of our stash repositories and jam these into gitea.

It's not an operation that i'll need to repeat, as far as i can tell in the documentation there's no procedure for doing this sort of thing.

I was thinking, as a feature, it might be handy to have an "upload zip" for migration to facilitate easy migration from other git servers.

I'm sure there're higher priorities, but figured it would make it easier for people to migrate and adopt gitea.

Thanks