Improve UX, add user guide for new users

[kartikcho]

Add improvements to the user experience like visual feedback on loading, a better more comprehensive user guide (read #74 for detailed context) and some other UI changes.

Steps:

• Add loading progress bar when processing large profiles (currently the app hangs and looks frozen).
• Add a modal to explain what the profiler shows, the different events and a legend key about the event colors.
• Re-design the landing layout