Performance: preload stylesheets

[kwajiehao]

Describe the bug
This isn't a bug, but a performance optimization. Google pagespeed shows us that our stylesheets aren't being loaded in an optimal manner - we should preload them instead. See a sample report here: https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fwww.vaccine.gov.sg

To Reproduce
Steps to reproduce the behavior This is a problem with how the template is loaded

Expected behavior
The stylesheets should be loaded as <link rel="preload" as="style"...> instead of <link rel="stylesheet"...>

Screenshots
N/A

[lisa-kwok]

adding onto the thread, there are two more lighthouse audit issues which are pending our investigation