* feat: added support mailbox to handle email channel (#140)
Added a new mailbox called 'SupportMailbox' to handle all the
incoming emails other than reply emails.
An email channel will have a support email and forward email
associated with it. So we filter for the right email inbox based on
the support email of that inbox and route this to this mailbox.
This mailbox finds the account, inbox, contact (create a new one
if it does not exist) and creates a conversation and adds the
email content as the first message in the conversation.
Other minor things handled in this commit:
* renamed the procs for routing emails in application mailbox
* renamed ConversationMailbox to ReplyMailbox
* Added a fallback content in MailPresenter
* Added a record saving (bang) versions of enabling and disabling
features in Featurable module
* added new factory for the email channel
refs: #140
