Add Systems and Theory as prerequistes to Applications - #111

Changed pages include:

• index.html - view
• 404.html - view
• blog/index.html - view
• coc/index.html - view
• computer-science/index.html - view
• contributing/index.html - view
• data-science/index.html - view
• faq/index.html - view
• getting-help/index.html - view
• math/index.html - view

There are more changes not included in the list