Mirror message sharing to IRC

[jvperrin]

Messages shared (from another channel or from past history) do not get seen in IRC, but they should be quoted or something like that to make it clear a reference is being made to a prior message.

[jvperrin]

Shared messages appear as this kind of message (with an attachment and is_share set to True):

2018-12-20 17:45:17-0800 [-] {'user': 'UXXXXXXXX', 'attachments': [{'is_msg_unfurl': True, 'author_icon': '[...]', 'color': 'D0D0D0', 'is_share': True, 'fallback': '[December 19th, 2018 1:37 PM] nattofriends: send the client quit thing to telnet, or maybe nc?', 'author_name': 'nattofriends', 'mrkdwn_in': ['text'], 'footer': 'Posted in #rebuild', 'channel_id': 'CXXXXXXXX', 'channel_name': 'rebuild', 'text': 'send the client quit thing to telnet, or maybe nc?', 'from_url': '[...]', 'author_id': 'UXXXXXXXX', 'author_subname': 'nattofriends', 'author_link': '[...]', 'ts': '1545255478.858300'}], 'team': 'TXXXXXXXX', 'text': '', 'event_ts': '1545356716.990600', 'channel': 'CXXXXXXXX', 'ts': '1545356716.990600', 'type': 'message'}