Support importing profile pictures from Slack

[wetneb]

When importing a Slack dump, profile pictures are not imported.

It would be nice to have support for this. The downloading of the images could be added to the slack-advanced-exporter (to match the workflow used for the attachments) or let mmetl download them directly on the fly.

[isacikgoz]

Thanks for the feedback @wetneb added this to our internal backlog!

[wetneb]

@isacikgoz do you have any thoughts on whether the downloading should happen in mmetl or in slack-advanced-exporter?

So far I have an implementation which does the downloading in slack-advanced-exporter, happy to contribute it as a PR.

[isacikgoz]

@wetneb probably slack-advanced-exporter would be the best tool for that as it handles all the export logic. mmetl is more like a conversion tool.