Allow to display messages from IMAP servers instead of syncing

[clochix]

Syncing messages between the remote IMAP servers and the Cozy database is hard. Some users have limited resources and would better just use the client to directly display messages from the IMAP server. Adding an option to allow such a behavior would be really nice.

[fairyfeller]

Take a look at Thunderbird options - https://support.mozilla.org/en-US/kb/imap-synchronization It sync message headers and also you can limit max size of content for downloading in KBs.