Make CI useful again and add type declarations

[GKFX]

This pull request:

• Adds type declarations to bring down the PHPStan error count
• Makes the CI not dependant on a PHPStan pass so that it can run through to the unit tests
• Adds alcaeus/mongo-php-adapter so that composer install can run without ext-mongo. This could be removed again if doctrine/mongodb-odm is upgraded, as in the elchris fork.
• Disables the Propel tests as all the Propel code seems to be missing something in the FOS\OAuthServerBundle\Propel\om namespace
• Updates the PHPStan configuration including the addition of phpstan-symfony.

The first commit is unfortunately rather large. A lot of it comes from running sed over Tests/ to add return type declarations, which inevitably produced a lot of changes.

[deguif]

@GKFX really nice, but could you split this PR in to smaller ones, please.