feat(content): Make mystery set strings translatable

Squashed and cleaned up version of
https://github.com/HabitRPG/habitrpg/pull/6367

test/content/mysterySets.test.js

@@ -0,0 +1,14 @@
+import {each} from 'lodash';
+import {
+  expectValidTranslationString
+} from '../helpers/content.helper';
+
+import mysterySets from '../../common/script/content/mystery-sets';
+
+describe('Mystery Sets', () => {
+  it('has a valid text string', () => {
+    each(mysterySets, (set, key) => {
+      expectValidTranslationString(set.text);
+    });
+  });
+});