Allow multi-database configuration via AR base model selection

[schmijos]

This is a proper follow-up to #129. We add two configuration options:

• abstract_model_class
  All explorative calls in this gem will be redirected to this class. Default is ActiveRecord::Base.
• available_abstract_model_classes
  If there is more than one entry, the GUI will allow you to select abstract_model_class. Default is [ActiveRecord::Base]

Remarks:

• I decided against reorganizing RailsDb::Connection and RailsDb::Database to keep compatibility for older Rails versions. This is also the reason why there must be two config options. The better way forward would probably be to look at the database configuration (ActiveRecord::DatabaseConfigurations) instead.
• This PR doesn't fix the fallback in lib/rails_db/connection.rb:7. It is still broken for Rails >= 6.1.
• bin/runsql and lib/standalone.rb will still use ActiveRecord::Base.

Impression: