adds slack link to side bar

[katyhuff]

Adds link to Slack to the side bar, as suggested by @gonuke during the presentation.
Removes google hangout link since I don't think we use it anymore.

Summary of Changes

One line, a link.

Design Notes

Retiring hangout link.

Check the box if your change does not break any of the following:

• API for Cyclus modules
• Input files
• Output tables for post processing tools

Related CEPs and Issues

None.

Associated Developers

@gonuke @munkm @dean-krueger

Testing and Validation

Not at all.

• I have read the Contributing to Cyclus guide
• I have compiled and run the code locally
• I have added or updated relevant tests
• I have added documentation for new or changed features
• This code follows the style guide
• I have updated the changelog

Reviewers, please refer to the Cyclus Guide for Reviewers.

[dean-krueger]

This still had the workshop link in it when I looked at it, but I just merged that PR so I think since this changes a different file this should be good to go and everything will work out. If it doesn't feel free to bully me and I'll fix it 😬

[dean-krueger]

oh but now it's out of date with main I guess? Paul always makes me fix this kind of thing, but also Paul (and certainly Katy too) is (are) much busier than I am, but I don't know if this is like a "formality" thing or like a "no it actually matters" thing so I'm just going to leave this as a comment and if in the future it's fine for me to hit the "update branch" button let me know and I will so!