[FIX] Importers not recovering when an error occurs

Improve how the importer progress gets sent to the client and change the structure of the importers imports

[FIX] Csv importer: work with more problematic data

Interface clean up, with users&channels being allowed to be empty for
CSV import.

* moving from coffee to js

* renaming method

* Continue work started by patcon in #2272 for importing slack again and again without duplicates. Currently errors out. Also fixed #2532 for Slack and HipChat.

* Ignore new slack channel format

* Add ability to upsert messages on sendMessage

* Show a better error for duplicated oembed records