Prevent Base64-encoded images from being sent to Elasticsearch

Currently, embedding the HTML text content of a Note document with an image results in its Base64-encoding being added to the document's note:note schema property - this field is always stored in Elasticsearch, resulting in a significant storage increase. This exposes a risk to the Elasticsearch cluster when multiple documents with embedded images are created.

To avoid this, Base64 content should be filtered out before requests to the ES cluster are made for Document creation/indexing.