[WIP] added back ICU classes from symfony/Icu

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #11447
| License       | MIT
| Doc PR        | n/a

The problem that this PR addresses is well explained in #11447 and the counterpart of this PR is the creation of the new `symfony/intl-data` repository, which is discussed in symfony/Icu#12. As explained there, there is a BC break as people will now need to explicitly add the `symfony/intl-data` dependency to get the ICU data.

I think we should insert a hard dependency for symfony/intl-data for all versions < 2.6. symfony/intl-data does not depend on ext-intl, so it's installable by anyone. Furthermore, the BC break (installing intl-data manually) is then fixed.