Allow Gogs to run from a suburl behind a reverse proxy. e.g. http://mydomain.com/gogs/

Conflicts:
	modules/setting/setting.go

Conflicts:
	templates/repo/release/list.tmpl
	templates/user/dashboard/dashboard.tmpl

Conflicts:
	routers/repo/setting.go
Merge pull request #463 from chadoe/urlroot

Allow Gogs to run from a suburl behind a reverse proxy.