Send notifications from template with email files

[CrystalPea]

In this PR:

When a notification is sent from a template that has email files, for each of these files:

• download the file from the S3 bucket
• send the file for upload to document download, along with the necessary metadata
• insert the resulting secure link to the file as a value into personalisation dictionary (where the placeholder key is the filename for the file)

Questions:

• how do we want to handle is_csv flag?

Thanks

Thanks to @quis for having a look while the PR was still a draft, and hashing out a simpler way to get the right email files in the comments.