When redirecting clean the path to avoid redirecting to //www.othersite.com (#5669) (Backport v1.6)

[zeripath]

Backport of #5669 to v1.6

Out of the box it is possible to get gitea to redirect to other servers:

$ curl -i --path-as-is http://localhost:3000//www.google.com/..
HTTP/1.1 302 Found
Content-Type: text/html; charset=utf-8
Location: //www.google.com/../
Date: Tue, 08 Jan 2019 21:53:05 GMT
Content-Length: 43

<a href="//www.google.com/../">Found</a>.

This PR cleans the path, prior to sending a http.Redirect.

Fix #5627

With thanks from @0x5c

Fix #5627

Signed-off-by: Andrew Thornton art27@cantab.net