More options to tag sent mails

It splits code of ArchiveSentMailsFilter into SentMailsFilter tagging mail and ArchiveSentMailsFilter which inherits from SentMailsFilter and additionally archives mails.

SentMailsFilter has additional option:

* to_transforms = `<transformation rules>`

 * Transform `To`/`Cc`/`Bcc` e-mail addresses to tags according to the
   specified rules. `<transformation rules>` is a space separated list consisting
   of 'user_part@domain_part:tag' style pairs. The colon separates the e-mail
   address to be transformed from the tag it is to be transformed into. ':tag'
   is optional and if empty, 'user_part' is used as tag.

 * It can be used for example to easily tag posts sent to mailing lists which
   at this stage don't have `List-Id` field.