cached event agenda is out of sync with legistar

[hancush]

on 5/15, an event agenda document was updated to reflect a withdrawn item, however the cached document was not updated and an out of date version was displayed in the web ui.

i did the following to resolve the issue manually:

• visit the event in question in the councilmatic ui and retrieve the ocd id from the console.
• using the ocd id from the console, visit the event in the ocd api and retrieve the agenda document url.
• in a python interpreter, generate the s3 key: import urllib; document_url = 'DOCUMENT_URL'; print(urllib.parse.quote_plus(document_url)) (here's the code that generates it in the pic app.)
• in the aws console, go to the s3 dashboard, then the councilmatic-document-cache bucket.
• search for the key you generated in your console, and click its filename.
• to the right of the "overview", "properties"... tabs, there's a latest version dropdown. click it, then click the trashcan to delete the cached image.
• visit the event in question in the councilmatic ui again, and confirm that document has been updated. you may see a red "invalid response" banner in the pdf viewer in some browsers. this is documented in PDF viewer: Browser compability #378. refresh again and confirm that the document has been updated.

it looks like the refresh_pic command tries to update cached documents for changed events, however the withdrawal of a matter on the event agenda did not register as a change to the event. so, we'll need to debug that and update the management command to reflect what we learn.

related to #347 and #407.

[hancush]

This management command has been totally refactored. This issue is superseded by #621.