When use builtin SSH server with concurrent operations, there are probabilities
One connection could use the command from another connections.

Fix this by set SSH_ORIGINAL_COMMAND for each command, not set in global scope.

Replace spaces with "%20" in "urlPrefix", before markdon processing.
The spaces were causing blackfriday (markdown processor) to behave
strange. This fixes #2545.

- Fix go-gogs-client#10
- Related to #809

Both are possible on explore and admin panel

Collaborators have write access as default, and can be changed via repository
collaboration settings page to change between read, write and admin.

- Allow to delete current avatar

- 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.