Using url_file_attachment user can specify if he like to change objects
like 'file_share' marked as external (in Slack servers point of view)
to attachments, so the only value would be remembered URL for the
"uploaded" document as an attachment. Or, treat it as is, and let the
slack-backup to produce file which contain the URLs and corresponding
local file names for such files.
Currently, if messages are generated using bot users, such messages will
crash slack_backup, since user.list API method returns only regular
users. In this commit there is detected a situation, where we have
'bot_id' in the data, and in case there is no such user in local
database, create it using data from bot.info API call.
If comment is sent by the user, different structure of the data is sent.
First of all, the type of this message is "message", but it contain
dictionary under 'comment' key, which can be confusing, which contain
needed data (like user id). For this kind of messages, in case of lack
of 'user' in main dict, dict['comment']['user'] will be used for getting
user identifier, while dict['text'] remains as a message text.
