Integrate webscreenshot with Slack

[mrakitin]

This is the update to post screenshots of the internal Sirepo servers to Slack (but can post screenshots of any page as well).

Links:

• https://github.com/maaaaz/webscreenshot to capture screenshots of the target webpages
• https://github.com/maaaaz/webscreenshot/wiki/Phantomjs-installation - renderer installation
• https://api.slack.com/apps - Slack apps
• https://slack.dev/python-slack-sdk/web/index.html#files - Slack docs for uploading files

[mrakitin]

cc @ahebnl, this is the update to post screenshots of the internal Sirepo servers to Slack.

Example: