Remove unused Bootstrap CSS

We import CSS from Bootstrap 3 via src/assets/css/bootstrap.css. That file is one of our largest assets. Bootstrap includes many components that we don't use in Frontend. We could reduce the size of the Frontend bundle by removing CSS from bootstrap.css that we don't use. We've already been doing that on a case-by-case basis. In some cases, CSS has been moved from bootstrap.css and integrated into app.scss. I think we should accelerate that process and aim to remove much of the CSS in bootstrap.css.

We already removed some Bootstrap CSS in getodk/central-frontend#44. However, I believe that that was reverted in getodk/central-frontend#199.