When redirecting, clean the path (#5669) (Backport to v1.7)

[zeripath]

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

$ curl -i --path-as-is http:https://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="https://www.google.com/../">Found</a>.

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

Fix #5627

With thanks from @0x5c

Signed-off-by: Andrew Thornton [email protected]