web: update branding colors and Sourcegraph logo images

Created by: valerybugakov

Context

Brand design has updated our logo and brand colors. We need to update them within the product and design system as well. See our documented brand guidelines here.

Relevant Slack thread.

Changes

• Updated hardcoded colors in the code.
• Updated home page SVG logos for both color schemes.
• Updated favicon.
• Updated browser extension icon-active-with-alert images. Figma still doesn't have an inverse mask feature 🙃
• Processed SVGs and images through minifiers: svgo and imgmin.
• Pointed window.context.assetsRoot back to local resources instead of https://sourcegraph.com/.assets in Storybook stories, to load updated assets.

Screenshots

Closes https://github.com/sourcegraph/sourcegraph/issues/22184.