Fix: filters bar appearing when there are no filters

Created by: attfarhan

Fixes https://github.com/sourcegraph/sourcegraph/issues/1764.

To fix the issue, I simplified how filters contributed from extensions are rendered by requesting Contributions in the SearchResults component and removing the SearchFiltersContainer component. The empty prop that SearchFiltersContainer had did not serve its purpose, as we cannot check whether a child component renders null from a parent component. Also, we never passed in the scope prop, so we didn't need to observe scopeChanges, which made the separate component unnecessary. LMK if I missed something @vanesa @slimsag.