SSBC: Allow to filter workspaces by ignored and unsupported

Created by: courier-new

Design for https://github.com/sourcegraph/sourcegraph/issues/34103.

When the user clicks the filter button to the right of "Displaying X of Y" on the Workspaces header...

(example here)

...they should have the option to filter out workspaces that are ignored (a .batchignore file was present in the repository) or unsupported (the code host that the repository lives on is unsupported by Batch Changes).

Open questions that should be solved by a design:

• Should these workspaces be visible or hidden by default?
  • In the preview list on the editor screen
  • In the list on the execution screen
• Is it important that we allow users to filter to only ignored/unsupported workspaces?
• How do we present the menu options in the UI when you click the filter button?
• Do we want to add any other filters on this menu?