Make nightly reports self-contained

Why

Nightly archive reports are missing css and icons, making them difficult to read.

Inlining all data makes the pages become fully self-contained, making them render properly in the archives without much setup.

Before: before.html

After: index.html

This pull request

• Replaces most icons with emojis (they were PNGs of mattermost emojis anyways)
• Inlines the stylesheets
• Uses base64 encodings of remaining icons to inline them
• Removes unused css blocks to cut total size in half