Add caching to profanity filter route

[JillianK]

This adds caching to the profanity filter route. Currently, we check webpurify each time a student uses a project with playSpeech in it for the first time. This will cache those results so we don't make as many webpurify requests for the same text.

Links

• slack thread about this

Testing story

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[breville]

Great work! 🤬