Basic background assembly implementation

[nonprofittechy]

Fix #788 fix #184

This refactors the document generation functionality in download_list_html into a new function, get_cacheable_documents, and adds some simple events that can be used for background assembly.

Additionally, it adds a single downloadable PDF row to the download screen.

This adds 2 tasks that are available for any bundle:

generate_downloads_with_docx_task and generate_downloads_task. Each task populates the _downloadable_files attribute of the bundle, either with or without the accompanying original DOCX files (when available).

test_aldocument_background_assembly.yml shows an example workflow for using the new background assembly functionality.

@KelizaMcGuire

[samglover]

Not sure I would spot a code problem, but it sounds like a good upgrade.

[ekressmiller]

@nonprofittechy, would it be possible to add the use_previously_cached_files option to send_email as well so that it could likewise use files generated in the background?

[ekressmiller]

@nonprofittechy, is there a way to use this method while still allowing an "Edit" button (taking you to the review screen) on the download screen?

I just tested:

1. arrive at the download screen
2. click button to go to the review screen and change a variable
3. resume interview, which returns me to the download screen

Since use_previously_cached_files is True, the change I made was not reflected in the assembled docs.

[nonprofittechy]

no, it doesn't work with the edit button. I suggest instructing the user to hit the back button, and making the previous screen a review screen that always appears