Change colour of numbers on pull request tabs if greater than zero

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

allow native and ssh-keygen public key check

Add ForegroundColor for labels to resolve #2033

Image attachments keep aspect ratio

Improved Pull Request refs

Docker Container update

- Remove the known issue about `.dockerignore` being ignored during DockerHub automated build as this has been fixed
- Added a note on the fact that we currently do no support building the container on RPi1

- Upgrade of gosu to v1.7
- Change in docker/build.sh to use `--no-cache` to prevent APKINDEX creation when installing dev dependencies
- Manual upgrade of Alpine on Raspberry Pi when building to make sure the environment is the same as the standard Dockerfile

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.

Test mailer button. Addresses #1531

test the latest version.

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.

Dockerfile cleanup for Alpine 3.3

Little refactoring of diff highlight.

Moving cache variable to template instead of in the struct.

Removed duplicate of paragraph

Also fix a possible race condition while install

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

Add AttributesInBind option in new auth source form.

LDAP: Fetch attributes in Bind DN context

Implement issue and pull request templates

This is useful especially to check whether we fetch right attributes, using
right LDAP search base and in right order.

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.

According to RFC 4511 4.2.1. Processing of the Bind Request "Clients may send
multiple Bind requests to change the authentication and/or security
associations or to complete a multi-stage Bind process. Authentication from
earlier binds is subsequently ignored."

Therefore we should not use 2 connections, but single one just sending two bind
requests.

- s6 is in main in 3.3, so we no longer need to mangle the repos file
- official image is periodically updated, so it's not preferred to do
  upgrades downstream (usually harmless, but inelegant)
- apk-tools in 3.3 supports --no-cache to avoid leaving the APKINDEX
  files in the image

Enable a way to checkout Pull Requests from remote refs

Also fix that no matter who, still able to see organizations with private membership.