Implement new attachment handlers

[khakers]

Adds an attachment handler interface and 2 new clients which implement it.

MongoAttachmentClient: stores attachments as binary data in a mongoDB document in the attachments collection
S3AttachmentClient: Stores attachments as S3 objects in an S3 service of the users choice.

[khakers]

download_attachment & delete_attachment are currently stubs in bot implementations, and I'm not sure they're necessary for the API at the moment

[khakers]

Should include tag for bot ID to help people that are sharing a bucket between multiple bots that they host

[khakers]

As a note for use, Cloudflare and Backblaze both offer 10GB free on their S3 cloud storage systems (R2 & B2, respectively). Cloudflare has no egress fees and Backblaze has some only if you don't route through a CDN and download more than you store.

[khakers]

Should be pretty close to final at this point.
I've implemented size checks so messages will fail to send if they have attachments that are too large.
The maximum attachment size doesn't get updated when you change the value via modmail without restarting.
The bot doesn't currently have a way to delete attachments, but that can be handled by a policy in your s3 server or manually for someone using mongodb storage.

Still iffy on the current s3 mongodb data format and if I should rename the mongodb datastore config value to "mongodb" from "internal" to reflect what it actually is.

[khakers]

This seems to be a decent bucket access policy
(allows only getting objects, no directory listing)

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": [
                    "*"
                ]
            },
            "Action": [
                "s3:GetObject"
            ],
            "Resource": [
                "arn:aws:s3:::modmail-attachments/*"
            ]
        }
    ]
}

[khakers]

Add noop attachment handler so the option to not save attachments exists.

[khakers]

Ability to set retention period for mongodb storage may also be desired