Spend points

[KimberleyEvans-Parker]

User Story:

As a user, I want some way of spending points so that I have more incentive to get them

Acceptance Criteria:

1. You can 'buy' or unlock something using points
2. The points decrease by the amount you spend
3. You are unable to use the item before you buy/unlock it

Why is the feature required?

Improves gaminfication

Describe the solution you'd like

You have locked items (e.g. themes) that you can' use unless you buy them using points

Additional Context:

You're welcome to come up with any type of item; themes is just an idea.
se701g2#235 in the main repo could be useful to have, but not needed
Linked with #11