Update register.js to allow shorter and longer usernames.

## Proposal:
Allow for two character and 16 character usernames. 

## Reason:
Shorter usernames are popular in many games that we see on the market today. Coupled with the fact that many other websites also allow them. Changing these values will allow for NodeBB to be unique in which usernames it accepts, as many website administrators turn to custom based solutions since other softwares (phpBB, vBulletin, IPBoad, XenForo) only support 3-16 character usernames.

#### I personally have 2 letter usernames I use. Would love to be able to sign them up :P