Improve Quality of HTML and CSS

[jwyatt1999]

For this PR I went through all of our HTML and CSS files and fixed any inconsistencies or issues I found. I 99% of the style from the html files into the css files and then organised the css files by page / logical group.

I have run the css files and generated html pages through the validator and they all passed.

I also added a few extra system test cases for the new leaderboard feature of showing the average scores.

I have also updated requirements.txt to match the current application requirements (I don't anticipate we will need to make any further changes to this unless we are adding new functionality).

HTML pages were validated by running application then pressing Ctrl + U on the page and copying the HTML into the w3 validator. CSS page was validated by throwing in the source code.

HTML pages validated:

• [x] Home page (logged in)
• [x] Home page (logged out)
• [x] Game page (daily)
• [x] Game page (freeplay)
• [x] Game page (test)
• [x] Credits page
• [x] Leaderboard page (daily, today)
• [x] Leaderboard page (daily, yesterday)
• [x] Leaderboard page (freeplay)
• [x] Login page
• [x] Profile page (daily)
• [x] Profile page (freeplay)
• [x] Register page
• [x] Rules page
• [x] 404 error page
• [x] 500 error page

CSS pages validated:

• [x] Stylesheet (there's only one)

Automated testing:

• [x] Unit tests all pass
• [x] System tests all pass

[jwyatt1999]

Hey @Bsubs , this will probably be the last PR (from me) which is actually delivering content. I've run all the validation, tests, etc. etc. and it all passed so this is ready for review :)

[Bsubs]

Looks good, splitting the css into 2 files is a freat idea for maintainability