Implement search filter for context identifiers

[harmitgoswami]

Fixes #3221

Excluded context identifiers by default when searching strings on the translate view. Now, there is a search panel, identical in style to the existing filter panel, which allows users to explicitly include context identifiers.

To reproduce results: Head to any string within the translate view upon running the server. Then, click on the magnifying glass icon on the right side of the search box. This action will open up the search panel, which currently only has one option. Either click the check mark icon to toggle the filter, or click the name of the filter and enter your search again (See note below).

Side note: The click behaviour for filters seems a little wonky, in both SearchPanel and FiltersPanel. I've ignored simplifying the behaviour for this PR.