Difficulty translation key (#1959)

Move the key to name mapping for difficulties into the translation
system.

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

src/client/components/Difficulties.ts

@@ -1,13 +1,6 @@
 import { LitElement, css, html } from "lit";
 import { customElement, property } from "lit/decorators.js";
 
-export enum DifficultyDescription {
-  Easy = "Relaxed",
-  Medium = "Balanced",
-  Hard = "Intense",
-  Impossible = "Impossible",
-}
-
 @customElement("difficulty-display")
 export class DifficultyDisplay extends LitElement {
   @property({ type: String }) difficultyKey = "";