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.

When repository contains a Slack type hook,
it changes original payload content.

This patch fixes it by using a local object to store
newly created Slack payload instead of assigning
back to the same variable.

- Update locales

- update required version of git-module for #2373