Use issue template form instead of template

New Slack and wording improvements in templates

NetBox should always be referred to as NetBox ...

Fix all instances of "Netbox".

```Shell
git ls-files -z . | xargs --null -I '{}' find './{}' -type f -print0 | xargs --null sed --in-place --regexp-extended 's#Netbox#NetBox#g;'
```

Ref: https://netbox.readthedocs.io/en/stable/development/style-guide/#branding

Change the phrasing of the alternative options

Gunicorn is replaced with nginx-unit ...

We now serve Netbox with an nginx-unit instance instead of Gunicorn.
This allows us to get rid of the extra Nginx container because Unit is
also serving the static files. The static files are now collected at container
buildtime instead of every startup.

Create config.yml for ISSUE_TEMPLATE ...

This commit adds links to the _New Issue_ section of Github.
The intention is to provide more hints where to get help and
further reduce the amount of Github issues that can not be
acted upon.

Update docker inspect command in bug_report template

Pull request template suggestion

Adds more labels to the Dockerfile ...

It also updates the README and the bug_report template to reflect
the changes. Additionally, in the README some relevant shields from
shields.io are added.

The labels follow [label-schema.org][lsorg] and the [OpenContainer
image spec, section annotations][ocis], specifications.

[lsorg]: http://label-schema.org/rc1/
[ocis]: https://github.com/opencontainers/image-spec/blob/master/annotations.md

A new issue category for feature/change requests

Update issue templates