Support CORS headers to http receive-pack/send-pack like gogs

[abhishiv]

• Gitea version (or commit ref): 1.5.0

Description

Hello, I would like to propose adding Access-Control-Allow-Origin header flag to gitea.

I'm in terested in this in order to get @isomorphic-git/isomorphic-git working with gitea in the browser. Gogs already supports it.

[http]
ACCESS_CONTROL_ALLOW_ORIGIN = *

...

Further background

cc: @wmhilton