Allow IP CIDRs in `visitor-request-limit-exempt-hosts`

[karmanyaahm]

For example, visitor-request-limit-exempt-hosts=matrix.me.org,1.1.1.1,172.0.18.0/16. Then, all of 172.0.18.0/16 will be exempt in addition to matrix.me.org and 1.1.1.1.

This will help with spantaleev/matrix-docker-ansible-deploy#2135, where there is no way to exempt requests from a whole network.
Is this feature fine with you? If so, I can make a PR within the next few weeks.

[binwiederhier]

This looks like a good feature and should be easy enough to implement I think.