changing the computed field setting needs to cause a field rebuild

Problem/Motivation

When the setting is saved on a field, the computed field doesn't show up until caches are cleared.

The form alteration needs to add a submit handler and do:

    \Drupal::service('entity_field.manager')->clearCachedFieldDefinitions();

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes