Use core methods for label/bundle keys, and add Media support

Problem/Motivation

The current hooks for node/term bundles can be condensed into a single hook using hook_form_alter() and an array of IDs to alter. Similarly the code in the validator can use core methods to be made generic without needing a switch(). This makes it easy to add support for new entity types.

Proposed resolution

Attached patch makes the form alter and field validator generic, and adds support for Media types.

Remaining tasks

Add an install file and update hook to convert old config schema re: taxonomy to new format - i.e. convert all unique_entity_title.settings.${vocab_name}_taxonomy_unique values to third-party settings on taxonomy.vocabulary.${vocab_name}