Add option to add/skip DMs

[volker-fr]

Currently the export option via CLI never shows direct messages. The web app itself shows always direct messages.

This PR:

• adds the option to show direct messages when exporting into a single file
• adds the option to hide direct messages when using the web app
• hides empty private channels, DMs & DM groups in the app.

[volker-fr]

@hfaran this PR should be good now. With the new template option the feature can now be used out of the box. Please let me know if anything is missing. Thanks.

[JeffMossMLW]

Hello. First off, the viewer is a great tool and much appreciated. In the past, I was able to see all DMs along with the public and private channels in Slack using the viewer. It's been awhile since I've needed to pull some info. Now when I run it, I cannot see the DMs. It only shows the public and private channels. I confirmed I am using the ver 2.1.0 and I am not purposefully hiding the DMs when I launch from CMD prompt. I also confirmed the dms.json file is in the zipped folder and it will open via browser. Any help would be greatly appreciated!

[volker-fr]

I created a fix for this this issue: #217

[JeffMossMLW]

Thanks! I upgraded to the ver 3.0.0. But the same issue occurs. The left sidebar navigation only shows public and private channels. No DM list or Group DM list. I see the dms.json file, but nothing shows up.

[volker-fr]

Accidentally copied the cli default over. PR to restore backward compatibility: #218

Workaround for 3.0.0 is to use --show-dms. I verfied this locally via python3 app.py -z ../myexport --show-dms.

There have been recently a few more feature flags, too. They might -- or might not -- be of interest. slack-export-viewer --help should show those and also the env var's tied to them.