Filter elements by metadata

A user should be able to filter a list of elements by metadata.

As the filter bar is getting overcrowded, I think we should implement new filters as modal, especially when they are complex like this one.

So the plan is to create a new vue.js modal that allows a user to:

1. select an allowed metadata or input a metadata name if they are an admin
2. select an operator (default to equal)
3. input a value
4. Hit filter

This would update the filters in use for ListElements, and update the display accordingly.

Bonus: display the current filter in the filter bar as metadata: name operator (value)