[#4284] Mark ext/intl as suggested only

- Marking as required in the framework makes it impossible to install
  the full ZF2 distribution when ext/intl is not available.
- Based on discussion in #4284, both zf2 and zend-i18n composer.json
  files should be in sync; otherwise, full zf2 will be installed if
  ext/intl is not available when attempting to install zend-i18n.
- Updated all code to raise a new "ExtensionNotLoadedException" when
  ext/intl is required but not detected.
Fix annotations

- `s/InvalidArgument/ExtensionNotLoaded/` in `@throws` annotations
Move ext/intl check to new constructor

- per @DASPRiD
Throw exception inside getLocale()

- instead of __construct, as ext/intl is only needed if no locale is
  explicitly set.
Merge branch 'hotfix/4284'

Close #4293
Fixes #4284 properly