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

Use hand-written SQL to do complex query

- 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.

Moving cache variable to template instead of in the struct.

Add AttributesInBind option in new auth source form.

This is feature is workaround for #2628 (JumpCloud) and some other services
that allow LDAP search only under BindDN user account, but not allow any LDAP
search query in logged user DN context.

Such approach is an alternative to minimal permissions security pattern for
BindDN user.

When <code> is nested inside <pre>, the next end tag token would not able to be the same
as outer-most start tag. So we only check outer-most start and end tag token to be the same.

Similar to GitHub:
https://github.com/blog/2111-issue-and-pull-request-templates

Priority:
- root
- .gogs
- .github

Remove the possibility of using email as user name when user actually push
through combination of email and password with HTTP.

Also refactor update action function to replcae tons of arguments with
single PushUpdateOptions struct.
And define the user who pushes code as pusher, therefore variable names shouldn't
be confusing any more.