Remove deprecated Network.Mattermost module

This change does a few things as part of removing the old server API
implementation in Network.Mattermost, which predates the "version 4" API
that Mattermost rolled out years ago:

- Removes Network.Mattermost. Import Network.Mattermost.Types instead
  for the types it was providing, and Network.Mattermost.Endpoints for
  the API endpoint functions.
- Adds an explicit export list to Network.Mattermost.Types.
- Adds an explicit export list to Network.Mattermost.Types.Base.

Author: jtdaugherty

mattermost-api.cabal

@@ -28,8 +28,7 @@ flag build-examples
   default:     False
 
 library
-  exposed-modules:     Network.Mattermost
-                       Network.Mattermost.Exceptions
+  exposed-modules:     Network.Mattermost.Exceptions
                        Network.Mattermost.Lenses
                        Network.Mattermost.Logging
                        Network.Mattermost.Util