Fix Main RECAPDocuments with attachment_number

[albertisfu]

This is a follow up of https://github.com/freelawproject/courtlistener/pull/4426

It requires the following issues to be addressed:

• Block PDF uploads with attachment number 0 and review whether we should avoid merging other attachment numbers into main documents.
• Prevent the extension from sending uploads with attachment number 0, and also review whether it's possible to avoid sending other attachment numbers for main document uploads.
• Clean up main documents with attachment number 0, and review whether it's necessary to clean up other main documents with other attachment numbers as well.
• For the remaining 395 documents related to this issue, we should review whether we can clean them up further to avoid future merging issues.

[ttys0dev]

I'm thinking...maybe we should be automatically crawling all the associated attachment pages(since they are free) when we get a document upload that doesn't have an associated attachment page uploaded yet. Probably an easy way to verify which documents are the main document.