Support breaking message body size into parts for Slack attachments.

[nzlosh]

The slack backend already splits up large messages into MESSAGE_SIZE_LIMIT for the send_message method. This patch uses the same logic for the send_card method.

[nzlosh]

This pull request is in response to an issue raise for the err-stackstorm plugin. fmnisme/err-stackstorm#16

[nzlosh]

@gbin @zoni Does this patch seem like an acceptable fix to be merged for the next point release?

[zoni]

@nzlosh this looks like a sane approach to me, I'd be happy to merge it. To help me verify this works correctly before I merge it though, could you possibly show me a minimal example plugin which:

1. Has a command to send a card which fits within the limit.
2. Has a command to send a card which gets split into multiple parts.

That way I can easily run it against a slack instance and see the behavior for myself.

[nzlosh]

@zoni I've written a simple plugin that will allow you test the multi-part message for cards. You can find the code here. https://gist.github.com/nzlosh/8230a9c41cdfb5d59df5845a390840dc

[nzlosh]

Something I noticed when testing was to post to #channel_name you have to provide 2 hashes ##channel_name. Without 2 hashes errbot's build_identifier reports an error. This is the same behaviour in the webclient under Linux and the native client under Mac OSX. From what I can tell the slack client is consuming the # before the message is being picked up by the bot.

[nzlosh]

I just saw the log in errbot's output:

15:20:38 DEBUG    errbot.backends.slack     Processing slack event: {'type': 'message', 'text': '.testcard <#C7SM7GMHD|ops> 1', 'user': 'U110FGZSQ', 'ts': '1509373238.000164
', 'source_team': 'T0V6H6HCJ', 'team': 'T0V6H6HCJ', 'channel': 'C7SM7GMHD'}

The channel is seen as <#C7SM7GMHD|ops> in the event. It looks like the slack backend may need to be updated to handle this format?

[nzlosh]

@zoni Did the gist example allow you test this PR?

[nzlosh]

@zoni or @gbin are there issues that are preventing this pull request from being merged?

[zoni]

Only a severe lack of time/energy on my part, for which I apologize. I'm going to try looking at this real soon. Please pester me again if I haven't gotten back to you on this in a week's time.

[zoni]

I finally got around to looking at this, and can confirm it works as intended. My apologies for not getting to it sooner. I'll merge this in now, the build failure should resolve itself as the cause of that has since been fixed on master.

Thanks!