[Validator] Support "maxSize" given in KiB

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | symfony/symfony#11027
| Applies to    | 2.6
| Fixed tickets | symfony/symfony#10962


To display the constraint validation message with an expected suffix, this PR add a new option in  File constraint.
Depend on the context:
- regarding the unit (MiB, km, kg...), they are prefixes
- regarding the value expected of the property maxSize (100M, 1024Ki) they are suffixes

BTW: at line https://github.com/symfony/symfony-docs/pull/3895/files#diff-aa9fd68c6e685f7e2773332c00fbcb68R179 it should be "binary and SI prefixes". But I choose to use the word suffixes to avoid user confusion...