Design-Debt Fixes

Created by: francisschmaltz

Small UI/UX fixes for design debt and design-related issues.

Fixes: https://github.com/sourcegraph/sourcegraph/issues/893 Fixes: https://github.com/sourcegraph/sourcegraph/issues/1111 Fixes: https://github.com/sourcegraph/sourcegraph/issues/433

Updated UI for file results make it clearer what expanding and collapsing 'more results' does and makes the header sticky to make it easier to browse files with lots of matches

Removes sidebar from org profile where it is superfluous

Updates sign up page on Sourcegraph.com to be more explicit that accounts are public.