Implement Push Rules API

[anoadragon453]

Spec: https://matrix.org/docs/spec/client_server/unstable.html#push-rules-api

Riot currently seems to break on refresh without this, with the error:

Unhandled rejection o/this.getPushRuleById@file:///home/user/code/riot-web/webapp/bundles/6213c8608d6e65e36a4f/bundle.js:34:30437
o@file:///home/user/code/riot-web/webapp/bundles/6213c8608d6e65e36a4f/bundle.js:148:18724
value@file:///home/user/code/riot-web/webapp/bundles/6213c8608d6e65e36a4f/bundle.js:148:19463
value@file:///home/user/code/riot-web/webapp/bundles/6213c8608d6e65e36a4f/bundle.js:1:23090
value@file:///home/user/code/riot-web/webapp/bundles/6213c8608d6e65e36a4f/bundle.js:1:22891
value@file:///home/user/code/riot-web/webapp/bundles/6213c8608d6e65e36a4f/bundle.js:1:21981
isEnabled@file:///home/user/code/riot-web/webapp/bundles/6213c8608d6e65e36a4f/bundle.js:15:21421
...

[babolivier]

Unsure whether this belongs to the clientapi component or needs its own.

[kegsay]

Probably its own to be honest. Lots of APIs, complicated regexp handling, isolated logically, etc.