Modular scss and make more use of bootstrap

[lamcw]

1. We should make use of sass and separate style.scss into several scss modules.
2. Use bootstrap class rules instead of using our own grid system.
   E.g., replacing,

.navbar {
  box-shadow: 0px 5px 5px 2px rgba(0, 0, 0, 0.2);
}

<nav class="navbar navbar-expand-lg navbar-dark">
  <!-- other stuff here -->
</nav>

with

<nav class="navbar navbar-expand-lg navbar-dark shadow">
  <!-- other stuff here -->
</nav>