- Remove unused config settings `[picture] service`

- Migrate: #2704 #2653
- Clone: #2701
- Mirror, Pull

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.

DisableSSH doesn't check the kind of ssh server to use, so that was
wrong. Use StartSSHServer instead.

The contants were placed in the same section as the scheme ones, which
may lead to confusion.

This commit adds the possibibility to use either the native golang
libraries or ssh-keygen to check public keys. The check is adjusted
depending on the settings, so that only supported keys are let through.

This commit also brings back the blacklist feature, which was removed in
7ef9a05588. This allows to blacklist
algorythms or keys based on the key length. This works with the native
and the ssh-keygen way.

Because of #2179 it also includes a way to adjust the path to
ssh-keygen and the working directory for ssh-keygen. With this,
sysadmins should be able to adjust the settings in a way, that SELinux
is okay with it. In the worst case, they can switch to the native
implementation and only loose support for ed25519 keys at the moment.
There are some other places which need adjustment to utilize the
parameters and the native implementation, but this sets the ground work.

Used by Android >= 5.0 to make the top bar colored.

Reference: https://developers.google.com/web/updates/2014/11/Support-for-theme-color-in-Chrome-39-for-Android

Local (DMZ) URL for gogs workers (such as ssh update) accessing web service. In
most cases you do not need to change default http://localhost:HTTP_PORT/. You
may need to alter it only if your ssh server node is not the same as http node,
eg. running behind proxy on different node than web server.

                     --- 80 public port -> 8080 -- web server node
                    /
    public proxy --<
                    \
                     --- 22 public port -> 10022 -- ssh server node

This option is not intended to be accessible via web GUI settings, since it is
unlikely someone needs to change it to somethings else than default
http://localhost:HTTP_PORT/ which should work for most of the cases.

But this should land into the documentation somewhere.

fixup

This is the old naming of the variable and I forgot to fix it. It now
works and is tested.

This allows the admin to disable the version information about gogs and
go in use in the footer.

This moves the minimum key sizes into the config file, so that anyone
can modify the restrictions.