Don't generate blurhashes at render time

Problem/Motivation

It seems like generating a blurhash is fairly resource intensive and significantly slows down response time.

Steps to reproduce

Proposed resolution

The hash should be computed when an image or media field is saved, and stored in the database. It should also be exposed as a basefield on file entities so that it can be fetched via JSON:API.

Remaining tasks

User interface changes

API changes

Data model changes