Improve the installation process

[provokateurin]

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Is your feature request related to a problem? Please describe.

Currently the installer is quite minimal, only allowing to configure the admin user and database.
Many admins struggle with creating new instances, because everything else has to be configured manually in the config.php. For that they need to look through the admin manual which is tedious and in some cases there are no good recommendations for different kinds of setups.

Describe the solution you'd like

The installer should become more interactive, such that more things like redis or imaginary can be configured.
Besides that settings like the default phone number region or maintenance window could be set (and which setup checks complain about if they are not).
We should ask for expected instance size, to be able to give suggestions accordingly.

We would need to collect a list of options that make sense here (probably going through the admin manual and maybe asking engineers which options are often used by customers). Of course we could implement every single of them, but that would be too much and make the installation process very unpleasant because you'd need to answer a huge amount of questions.

Then a modular system like setup checks should be put in place, so that we can easily add new options later and also re-use the same information both for the CLI and web installers.

[sorbaugh]

@marcoambrosini this could benefit from a Design review