Add API endpoint for updating a user's profile

[timmyichen]

Project: #19
Depends on: #8

Description
This issue adds an API endpoint for updating a user's profile.

Add an API endpoint:

• PATCH /api/users/:id

This endpoint should allow the updating of data for the user's model, limited to the following fields:

• bio
• social links added in #8

The endpoint should throw a ForbiddenError if it's called on any user except for the currently-authed user (i.e. people can only edit their own profiles)

Partial updates should be allowed; i.e. not every field above is required when calling this endpoint.

User-submitted data should be sanitized to not include markup (e.g. can't set your bio to be <script>alert('hi')</script>)

Acceptance Criteria

• API endpoint exists
• New acceptance tests exist and pass (covering edge cases)
• Existing tests pass