[web] Restyle filters button
Restyling the filters button. Note that in the comments, it has been decided that we'll use a different icon than in the designs.
For now keeping the button after opening the filter panel. This will be changed in the future
Test Plan: Run web app, checked that the button displays correctly, and pressing it opens the filter panel
Reviewers: michal, kamil, kuba
Reviewed By: kuba
Subscribers: ashoat, tomek, atul
Differential Revision: https://phab.comm.dev/D6740