Email headers can be encoded with different charsets, which is signalled
using a special character sequence. The go-message package provides two
different methods for accessing header values, Get(key) (actually
inherited from the embedded textproto.Header) which returns the raw
header value and Text(key), which returns the header's value decoded as
UTF-8.
Before, in the maildir backend, we were using the Get method which
sometimes resulted in encoded headers being displayed in the UI. This
patch replaces the incorrect usage of Get() with Text().
Add the initial implementation of a backend for Maildir accounts. Much
of the functionality required is implemented in the go-message and
go-maildir libraries, so we use them as much as possible.
The maildir worker hooks into a new maildir:// URL scheme in the
accounts.conf file which points to a container of several maildir
directories. From there, the OpenDirectory, FetchDirectoryContents, etc
messages work on subdirectories. This is implemented as a Container
struct which handles mapping between the symbolic email folder names and
UIDs to the concrete directories and file names.
