Add basic rate limit implementation

[jjnesbitt]

Partially addresses #1902

This PR adds basic rate limiting via DRF throttling. This is a very permissive version of rate limiting, imposing a rate limit only for anonymous users. This limit is also purposefully set above any number I could obtain through local usage of the GUI. The point of this first step of implementation is not to be a full solution, but to protect against the most extreme of use cases, with no impact on existing legitimate users, anonymous or otherwise.

This is very similar in implementation to a previous attempt we had made to introduce rate limiting, although that was sort of made in response to a crisis. The current circumstances and implementation are slightly different, in the following ways:

1. The crux of the issue in the previous implementation was that the asset list endpoint was very slow, which meant that a trivial amount of requests to it could cause a DOS. Rate limiting cannot prevent this effectively. The asset list endpoint has been fixed, so this limitation no longer exists.
2. The previous implementation only applied to the asset list endpoint, while this implementation applies to all API requests.