Optimize Thought and Toolbar components

[anmolarora1]

Addresses #484

It was observed that on every cursor move (up and down) the toolbar was gratuitously re-rendered, which caused performance overheads. After moving the cursor-dependent logic to mapStateToProps, the rendering time was reduced by ~20% (for revisited thoughts)
All the thoughts were re-rendered because of similar computations placed inside Thought.js. Since the connect function implicitly wraps components inside a 'memo', the values that were previously calculated in the component could be pre-computed and passed down as props, resulting in another significant reduction in rendering time(~30%)

On average, the rerender time has been reduced to ~50% (reports attached)
profiling_data.zip

If the logic is moved to selectors it'd result in better readability, separation of concerns, and possibly, further optimization(if we use reselect/custom memoizing functions).

[raineorshine]

Fantastic work. Thanks so much!