Indexing of files/custom events

Seshat need not conern itself with media but it'd be nice if a client could say oh this media event is actually this blob of text which I fetched from the media store, so index that text as well as the `body` of the media event (filename).

TL;DR: `ability to pass additional terms to index with an event for events which seshat could handle better, e.g media`

Start by tracing how Seshat indexes an event and how the media event's body, described as the filename, is handled. Determine where a client could provide additional terms for indexing, then verify that those terms and the event body are both searchable. The issue names no files or tests, so locating the relevant indexing entry point is part of the work.

Written by the indexing model from the issue text.