Discussions were being toggled by jquery DOM querying them and toggling
visibility but in vue, only the open discussions will be in the DOM
Fix includes:
 - Adds a getter to the store to get the expanded discussions
 - Adds an action to collapse a discussion
 - When the user clicks the button, all data needed is now accessible
   through a getter and we can dispatch an action to toggle the discussion
   within the state, instead of showing/hiding with jQuery
 - Removes hardcoded properties

Resolves #48237

Removes double export for actions in diffs module in mr refactor