Lower the required usernames length from 5 to 2

Both the previous and the new value are completely arbitrary but some
users suggested that we reject names like "Li" for no good reason,
especially on private boards, cf #202.

Hopefully one day we'll have an administrator configuration panel.

Fixes #202
Closes #210

client/config/accounts.js

@@ -5,7 +5,7 @@ AccountsTemplates.addFields([{
   type: 'text',
   displayName: 'username',
   required: true,
-  minLength: 5
+  minLength: 2
 }, emailField, passwordField]);
 
 AccountsTemplates.configure({