Refactors core domain logic into new package

- Refactors web/handlers.NotifyHelper and
  web/handlers.NotifyResponseGenerator into components:

  * Courier:
    - responsible for main workflow
    - receives collaborators as arguments to its constructor
    - executes workflow by calling Dispatch
    - returns postal.Response objects that can be marshalled to JSON

  * FileSystem:
    - confirms existance of files
    - reads files in as strings

  * Mailer:
    - delivers messages to all users in a given request
    - delegates to mail.Client to send individual messages

  * MessageContext:
    - contains all context information for sending a message
    - handles escaping HTML content

  * Packager:
    - compiles the message context into a deliverable mail.Message

  * SpaceLoader:
    - loads the space and org info from CC

  * TemplateLoader:
    - loads templates from the file system
    - handles overridden template loading

  * UserLoader:
    - loads the users from CC and UAA

- Other miscellaneous changes:
  * replaces magic strings with const values
  * postal package nolonger writes directly to a http.ResponseWriter
  * postal package no longer parses the http.Request parameters
  * postal error types are derived from strings
  * moves files_utilities package functionality into postal.FileSystem

[#75908058]