Slack channels should still import message text/content if attachment size is too large

Currently we fail to import messages if the message contains a file over 25MB. However, it would be nice if we dropped the attachments that are too large, and still imported the message text. When this happens we should also display some type of warning to the end-user that the message contained an attachment that was dropped due to the 25MB size limit.

The current flow can be quite the negative experience for customers as they don't quite know why the messages aren't appearing in Front.