Repo setting to delete and disable wiki

- Remove unused config settings `[picture] service`

Set DefaultBranch to master when importing a new repo.

update README.md

ISSUE_TEMPLATE: suggestion to test at try.gogs.io

I've noticed that a lot of issues cannot be reproduced on http://try.gogs.io,
which either hints about specific database type problems or
hints about bugs which are already solved in the newer version
(as http://try.gogs.io is usually a newer build).

This patch adds the suggestion to test the issue at http://try.gogs.io in
the Github "issue template". The user can answer: "Yes", "No", "Not relevant".

"Not relevant" is an option where testing on http://try.gogs.io makes no sense as
the bug is unrelated to the Web UI or is very specific in nature.

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

Hide the download archive button when viewing wiki pages

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

minor grammar fix

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.