Add system notes for changes to Linked Resources widget

Proposal

Add system notes for creating and deleting items from the Linked Resources widget.

Format for adding:

@username added a <link_type> link --> and populate link type with Zoom or Slack, if we detect that the link is to either of those places

@username added a resource link --> if it's not a Slack or Zoom link (for everything else)

Format for deleting:

@username removed a <link_type> link --> and populate link type with Zoom or Slack, if we detect that the link is to either of those places

@username removed a resource link --> if it's not a Slack or Zoom link (for everything else)