Discontinue shipping qan-app with pmm-server (v2.X)

Goal:

We discontinue shipping the previous version of QAN (v1) along with pmm-server. The previous version of QAN used to be accessible on the `/qan` route in PMM.

Description:

To that end we want to do the following:

• No longer bundle the qan-app assets (code) with pmm-server

• Remove the `/qan` route from nginx.cong in pmm-server

• Update the pmm-server build scripts

• Archive the repository /percona/qan-app

• Update our documentation, including old images and READMEs

How to test:

• http://pmm-server/qan should result in a 404 http error

• assert that all tests pass

Implementation details:

The following repos may be involved:

• https://github.com/Percona-Lab/pmm-submodules

• https://github.com/Percona-Lab/percona-images

• https://github.com/Percona-Lab/jenkins-pipelines

• https://github.com/percona/grafana-dashboards

• https://github.com/percona/pmm

• https://github.com/percona/pmm-server

• https://github.com/percona/pmm-server-packaging

• https://github.com/percona/pmm-update

• https://github.com/percona/pmm-managed  

• https://github.com/percona/qan-app (archive)

This will probably require the devs to delete the submodule qan-app in their local forks of percona-lab/pmm-submodules: