AddCSRFexemptroutesupporttomiddlewareextender

Fixes #1941, #1905

Part of #1891

Changes proposed in this pull request:
- Add an addCsrfExemptRoute method to the Middleware extender
- Improve Middleware processing system to support this

As per https://github.com/flarum/core/issues/1941#issuecomment-586217862, CSRF exemption is likely to be one of the most common use cases of the middleware extender. As such, considering that there isn't a lot of complexity, it might be easier to implement on our end instead of forcing everyone to override the CheckCsrfToken middleware.

TODO:
- Add integration tests for this (will be done after /core decides which extender this should be a part of).

Reviewers should focus on:
1. Should this be a separate extender? I thought that since it affects middleware, and we already have lots of extenders planned, it would be best to have it as a part of the middleware extender
2. Is the mechanism for providing the exempt paths to the CheckCsrfToken middleware the best approach possible? I modeled this largely off of the Routes extender
3. Code style?
4. Is the mechanism for matching paths to exempt routes what we want? (currently done via wildcards).

Confirmed
- [x] Backend changes: tests are green (run

).
- [x] Backend manual testing

该提问来源于开源项目：flarum/core