Feature: Add "Select all" in sources filter

[calumbell]

It would be good if we could upgrade the SourcesModal component to include a piece of UI that allowed users to select/deselect all sources.

I imagine something like a mixed/tri-state checkbox that can be checked, partly checked, or unchecked.

It is checked when all sources are selected. Clicking it will unselect all sources.
It is unchecked when all sources are unselected. Clicking it will select all sources.
It is partly checked when at least one, but not all, sources are selected. Clicking it will select all sources.

The only file that should need to be changed should be /components/SourcesModal.vue, although implementing the tri-state logic may make most sense in its own component