* Move session validity check and session author ID fetch to a
separate function. This separate function can be used by hooks,
making it easier for them to properly determine the author ID.
* Rewrite the remainder of checkAccess. Benefits:
- The function is more readable and maintainable now.
- Vulnerability fix: Before, the session IDs in sessionCookie
were not validated when checking settings.requireSession. Now,
sessionCookie must identify a valid session for the
settings.requireSession test to pass.
- Bug fix: Before, checkAccess would sometimes use the author ID
associated with the token even if sessionCookie identified a
valid session. Now it always uses the author ID associated
with the session if available.
