Page lags when replying to a thread with a lot equations

[karuboniru]

Cause: https://github.com/StarsRivers/Mathplus/blob/be09c9a06b68255e5dc66fcde82f890bb767d58b/js/src/forum/index.js#L82-L100
On every change (~100 ms) in comment box, all MathJax objects will be re-rendered, which can be very painful.

[karuboniru]

Closing as #21 is merged.