New `save-palette` backend route

User Story:

As a developer, I want to be able to hit a backend route to add a palette to a user, So that our users data is saved to the DB and can be synced

[ ] Create a new PUT route in index.js at the path "/save-palette"
[ ] Make sure that the callback function is async. (Here's a good reference for this)
[ ] If req.user exists, then we want to use the findOneAndUpdate method on our User DB model
- [ ] Use this documentation to build our findOneAndUpdate DB query.
- [ ] Because palettes is an array, we can use the mongo $push operator to append our new palette to the array rather than replace the whole thing.
- [ ] We also want to return the updated document from this route so that we can immediately display the newly added changes in the browser
[ ] Use Postman to test this new api endpoint (remember for this to work you'll need to be logged into Halogen or else there won't be a user in req.user).

Note: When creating a REST API, the PUT route is used to update an existing document, while POST is used to create a new one.