**Summary**
Install new `@apply_metadata()` on HTML and remove decorators from
delegating partitioners EPUB, MD, ORG, RST, and RTF.
**Additional Context**
- All five of these delegating partitioners delegate to
`partition_html()` so they're something of a matched set. EML and MSG
also partially delegate to HTML but that's a harder problem (they also
delegate to all other partitioners for attachments) that we'll address a
couple PRs later .
- Replace use of `@process_metadata()` and
`@add_metadata_with_filetype()` decorators with `@apply_metadata()` on
`partition_html()`.
- Remove all decorators from delegating partitioners; this removes the
"double-decorating".
