Improve check if Shopware is really completely installed

[mitelg]

Right now, system:is-installed only checks if the migration table is present. if so, the setup script assumes, Shopware is completely installed without issues.

This assumption could be wrong, as the migrations could not be completed, or a admin user is missing, or the SalesChannel was not created, or plugins were not installed completely.

It should be possible to verify each necessary step and if fails, to correct it.

e.g. sales channel:

• check if sales channel with given APP_URL is existing
  • if not, check if sales channel exists with previous APP_URL
    • if yes, update this sales channel with new APP_URL
    • if not, create new sales channel

To achieve this, some new commands need to be introduced in the Shopware core

[mitelg]

maybe the new health check system can be used. implemented as Readiness checks