Improve test cases, config settings, also show SSH config settings on admin config panel.

Config option [server] SSH_LISTEN_PORT to the port the builtin SSH server will be listen.
It can be different from SSH_PORT which is supposed to be exposed in the clone URL.
This should solve the problem when user runs Gogs inside Docker container
and still want to use builtin SSH server.

Make it easier to keep track of upstream changes and bug fixes

- #1496: fallback plain text
- #1002: add date header
- #913: fix encoding of header

Signed-off-by: Unknwon <u@gogs.io>

The same logic was duplicated in three places. Factor it
out so that we can add further fallbacks in a single place.

- .gopmfile: update all commits

- routers: use new binding convention to simplify code
- templates: able to set HTTP port number in install page