Standardise use of prettier tool

[nmcharlton]

We have the prettier library configured in the project but its use is not automatically enforced and it is not set up consistently across server and client (in */.prettierrc).

We need to do the following:

• Apply our style guide consistently across the project (as described in the README)
• Add a pre-commit hook to run prettier src --write in client and server to ensure the style is enforced