Add an API Route for password change by the user

[siddnikh]

Need to make an API route that lets the user update their password. Needed to solve this issue: coronasafe/care_fe#2459

[sainak]

@siddnikh we can just fix the UserSerializer update method to handle password field changes, also make the password a writeonly field, by doing this we can just PATCH the password, I don't think there is a requirement for a separate endpoint.

[siddnikh]

@sainak I ended up making another route because I was a little unsure about how to proceed. Please review it when you get a chance and tell me if there's some changes I need to add to do it how you suggested to.

[siddnikh]

Here's the PR that solves this issue: #797