FEDX-24

1. Extract the full sentence rather than parital of a sentence, so that languages with different word orders than English can benifit from this.
2. Using ngettext instead of gettext with 'xxx(s)', as Transifex treats `()` as special characters so that it enforces the translated string must have the same number of `()` pairs. Such enforcement makes translations difficult, as not every language can use forms like 'xxx(s)' to represent both singular & plural within a single word.

* updating vendor files
* updating class syntax (to new FA-based classes) for all UI elements
* correcting broken tests

This commit adds validation for course advanced settings. Currently when course
administrators make invalid changes in the Settings/Advanced Settings tab,
they're not notified through a new modal window of the list of invalid settings
changes.

* Extending CourseMetadata
    - Previously, we only had update_from_json method in CourseMetadata.py,
      and it was only validating one field every POST request.
    - Now we have validate_and_update_from_json method that encapsulates the
      functionality of update_from_json into a validation call
    - To avoid discrepancy of validation standards between modules, validation
      uses the from_json method implemented to each field in xblock.

* Different Response in advanced settings ajax requests
    - After receiving a POST ajax request, course.py calls
      validate_and_update_from_json, and sends a json object of either:
        1) valid course metadata model
        2) error objects

* Error Messages shown in validation-error-modal
    - error objects passed through ajax are shown in a separate modal.