Add metrics about attachments and email storage size

[devmount]

Is your feature request related to a problem? Please describe.
@drfedtke stated in #182, that it would be nice to have additional metrics about attachments and the storage size of emails.

Describe the solution you'd like
There are several possibilities. Most obvious would be the total number of emails with attachments or the total size. Also possible is the average number of files attached or the average size of an attachment.

Describe alternatives you've considered
In fact, every existing chart in ThirdStats is currently based on mail count, but it could also just be based on mail size. So in theory, there could be a switch on top where the user can toggle between showing stats based on mail counts or based on mail size.

Additional context
#182

[devmount]

Thunderbirds WebExtension APIs currently don't provide a corresponding attribute to retrieve information about attachments, so for now this will be on hold. In the meantime I encourage you to discuss the proposed metrics about attachments.

[devmount]

Thunderbirds WebExtension APIs currently don't provide a corresponding attribute to retrieve information about attachments

Correction: it doesn't exist in the message header, which ThirdStats only processes. It does exist in the messages body, but I avoid accessing that for privacy and performance reasons.

See this discussion thread.

[devmount]

The size attribute is now implemented in messages headers since TB 90
https://webextension-api.thunderbird.net/en/latest/messages.html#messages-messageheader

[graytape]

Is there any update?
I just installed the night build, but it doesn't seem to show anything like that yet

[devmount]

Not yet. Adding new metrics depends on some deeper UI changes to keep things clean and organized. Feel free to take part in the discussion: #376