Migrate to pure components where possible

[brendo]

This MR cleans up the components to be pure where possible. It reduces the amount of wasted time reported by the Perf tools down from 27 elements to just 8.

By accident, this also improves #31, as the active item will now update as the user scrolls down the page. Note that it will not automatically expands the tags, but that should be possible!

• We had a lot of unnecessary binds in constructors. This only happens when the context actually changes before the function is called, such as an event handler.
• setState automatically merges state, there is no need to use destructuring.
• Make better use of destructuring for brevity.