Append limit and quota support

Each account has 3 configurable limits:
* Max size of an individual message
* Max amount of messages
* Max total size of messages

It should be possible to define global default values for all 3 of them while overriding them on per-account basis if needed. There is already folder.Namespace structure that is embedded into Account model. It should be enforced by refusing to write messages that would cause either limit to be exceeded. Inability to check limits should lead to fail-open behavior - message should be written either way.

In IMAP terms, account has a single quota root. Though this may be affected by #9 in the future.