Before downloading we show the encoded size of attachments. After download we
strip the transport encoding to find out the size of the decoded content.
We have been throwing away all attachments already, so it doesn't make
too much sense to keep local messages. And when we're not keeping local
messages we can remove all entries from the 'threads' table.
This change breaks all kinds of things, e.g.
- deleting messages
- updating messages
- downloading attachments
- deleting attachments
- searching in message bodies