Avoid out of bounds error by using strings.HasPrefix to check for
starting SSH2 text rather than assuming user input has at least 31
characters.

Add tests for bad input as well.

Fixes #8800

* Make PEM and SSH2 keys work

* add ssh2 testcases and PEM cases - and fix PEM

* Add final test to parse the proposed key

* Strip newlines from SSH keys before adding them

Fixes: https://github.com/go-gitea/gitea/issues/7500

* add test for CheckPublicKeyString

* add one more test

* simplify test

* further simplify

* make fmt

* Use go method to calculate key fingerprint

* add gitea copyright

* use native go method only for built-in server

* refactor and add tests

* add gitea copyright

Makes it possible to set the work path as a CLI option instead of
relying on environment variables which are somewhat opaque

- Update import paths from github.com/go-gitea to code.gitea.io
- Fix import path for travis

See https://docs.travis-ci.com/user/languages/go#Go-Import-Path

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

TravisCI is too old for ed25519, so it can't be tested correctly.

The old API was using []byte, but was changed to string without running
the tests again.
It also sets the variables from the configuration to make them work.
Maybe there is a better way to do this.

This commit adds the possibibility to use either the native golang
libraries or ssh-keygen to check public keys. The check is adjusted
depending on the settings, so that only supported keys are let through.

This commit also brings back the blacklist feature, which was removed in
7ef9a05588. This allows to blacklist
algorythms or keys based on the key length. This works with the native
and the ssh-keygen way.

Because of #2179 it also includes a way to adjust the path to
ssh-keygen and the working directory for ssh-keygen. With this,
sysadmins should be able to adjust the settings in a way, that SELinux
is okay with it. In the worst case, they can switch to the native
implementation and only loose support for ed25519 keys at the moment.
There are some other places which need adjustment to utilize the
parameters and the native implementation, but this sets the ground work.