Bumped Bootstrap for 3.4 -> 4.3
Added popper.js library which is a dependency on Bootstrap 4.x

Some big changes:
- all .visible and .hidden attributes are now removed and replaced by
 d-{breakpoint}-{value} style classes which break at the "breakpoint" and they
 can be hidden by "value" of "none".

- grid values are now replaced by sass mixing $grid-brakpoints

- @media queries are all replaced by @include queries

- All sub-items of navbar-nav now need nav-item and all <a> tags need nav-link.
- "active" flag needs to be added to `<a>` tag inside nav-item

- btn-default is now replaced by btn-secondary and btn-default is now gone.
- btn-outline-secondary is being used at many places too.
- background colors can be replaced .bg-{color} attributes.

- Grids are now slightly different, we have changed the breakpoitns to align
  with default bootstrap ones

  tn -> xs
	sm -> sm
	md -> md
	lg -> lg
	   -> xl

TODO:
- Thread list still doesn't look like it should
- Button colors are still a bit off in many places, btn-secondary needs some
  replacement color
- Navbar height is more and needs better alignment for options