Disallow abstract definitions from doctrine event listener registration
ISSUE #11300: Improve Routing Syntax Import Error

modified FileLoaderLoadException message to present the previous (Parse)Exception first
Add test and small code fix
Addition of the symfony license text
- Removed spaces around the concation dots to be more consitent
- adjusted some formatting
Merge remote-tracking branch 'upstream/master'
removed some more spaces
[Validator] Fix constraint violation message parameterization
[Validator] Fixed value-to-string conversion in constraint violations
[Validator] Simplified IBAN validation algorithm
[Validator] Fixed and simplified IsbnValidator
[Validator] Simplified IssnValidator
[Validator] Simplified and explained the LuhnValidator
[Validator] Added "{{ value }}" parameters where they were missing
[Validator] Fixed date-to-string conversion tests to match ICU 51
[Validator] Fixed CS
[Validator] do not call getter inside ExecutionContext to prevent unnecessary calls
[Validator] Renamed valueToString() to formatValue(); added missing formatValue() calls
properly handle null data when denormalizing

If null is passed to denormalize(), no property values can be set on
the denormalized object. Additionally, this fixes passing values to
the denormalized object's constructor if the incoming data is an object.
[Console] Skip commands from ConsoleCommandEvent

Use case: We have different variations of the same application, for which
only certain commands are allowed. Right now this is done in a custom
Application class, but it would be much easier to just be able to skip
commands from a listener, where you can disable commands via the Event
object.

This patch provides this feature and corresponding test cases.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes - for Console tests
| Fixed tickets | None
| License       | MIT
| Doc PR        | None