introduce test_factories

Conflicts:
	idt_slack/core/tests.py
refactor
refactor tests
support for custom icons and parse style in post_slack_message
dones_stream command
introduce attachments support/format
initial command code
reformat dones_stream command
add slackteamstate.last_imported_done_created field
implementation of cutoff value update flow
remove code duplication
some style improvements
err on the side of high page_size API calls
Merge branch 'v2-dones-to-slack'
fix ordering flaw in dones fetching logic
improve style
fix logging
fix like/comment/permalink URL
Merge branch 'master' into celery