Updating the Look of the Fiji Website

[mattxwang]

This PR implements the changes discussed in #2. Specifically, it:

• Minorly changes the layout and flow of the site's content
• Updates Bootstrap to Bootstrap 4, which involves:
  • Changing outdated components to new ones (e.g. well to card)
  • Updating existing components (e.g. buttons and the navbar)
  • Uses Bootstrap's flexbox-based grid to position items instead of tables
  • Updates the jQuery dependency to jQuery 3
  • Adds the now-required popper dependency
• Adds FontAwesome for brand and decoration icons
• Adds automatic OS detection for the "Downloads" section
  • Displays an error if on a non-standard OS
• Changes how the plugins informational pane works:
  • Now uses a Bootstrap tab-pane instead of a Bootstrap carousel
  • Now takes 6 random plugins from plugins.json, rather than having hard-coded entries - allows for future users to add and remove plugins easily
• Adds a quick README to introduce users to the repository

[ctrueden]

Thank you so much for working on this, @malsf21! 🏆