03/13/2023: Slack clean up and updates

[jshinm]

• Change slack welcome message

  • Welcome to the Biomedical Data Science @jhu Slack Workspace! You are now in the #general channel. If you would like to communicate with Jovo, just..... (please fill in the blank for how to DM me directly).

• Clean up slack members

  • convert everyone who is not a jhu email address but is a regular member or a multi-channel guest to a single channel guest, with #guests being their only channel.
  • everyone who is invited (but not yet accepted), please revoke their invitation
  • https://neurodata.slack.com/admin