[Bug] Plugin overrides stardart Sylius behaviour that breaks custom Sylius installation

[dbalabka]

SyliusPayPalPlugin version affected: 1.7

Description
I've found that the plugin overrides some of the standard Sylius behaviour that conflicts with already existing customization.

There are a few tickets that report the same issue:

1. [Bug] Default Payment Method #302
2. Got 500 in select payment #154
3. [Bug] Remove final from controller #301

Possible Solution

1. I propose changing the way the plugin integrates into Sylius and utilising the thin extension points instead of overriding Sylius's behaviour.
2. An alternative solution might be providing the possibility of opting in to use overrides.
3. Create a separate plugin that provides thinner Payum integration for Sylius