Responsive CSS for Mobile Devices

[0hAodha]

At present, the CSS is not responsive for mobile device screens, resulting in some spacing-related problems:

• The biggest one is the navbar at the top of each page: the spacing is wrong and the "Tags" link goes off the screen.
• The "Recent Posts" Section has the dates of each post on the following line, resulting in an unpleasant flow. Perhaps remove date entirely on mobile as space is at a premium?
• The table in the "Fretboard Memorisation with Modular Arithmetic" article goes off the screen.
• Ordered & unordered lists are generally too narrow to read comfortably, could make better use of space and have less whitespace padding.

[0hAodha]

Fixed:

• [X] Navbar spacing.
• [X] Tables overflowing (fixed in 7dbfa6d7245fe385c4209237b855f24b01c183b1).
• [X] List padding.
• [ ] Publish dates overflowing

These issues would've been better off listed separately, so I will be moving the date overflow issue to its own issue until its resolved, and I will close this one as it is 75% resolved.