Enhanced error message flow

[tracey-le]

just extra ideas after main things for #18 (Plain error messages) have been done:

• showing positive feedback when you have a valid username/nickname/email/password on the signup page
  
  (see sign up page here for idea https://try.discourse.org :P) e.g. 'Username is available' 'Valid password' 'Valid nickname'

• if reload page for an error message, retain values of any other input fields.
  
  (For example, if you started filling in the username, nickname or email fields on signup, it should get that information and set the value of the input fields to it.
  
  Same with for asking question: if the page reloads because file not uploaded or because file type not supported, if you started entering a post title and description, we could make re-set the value of the input fields to keep those.)

• Would be great to change the flow so that you can start a comment without being logged in, and then still be able to post it once you do log in.
  
  (again, retaining the value of the input fields — so if you started typing a comment, it would be a nice extra if the field still had what you started typing when you came back to the page.
  
  also would be a cool extra if the site redirected you back to the page you wanted to comment on after you signed in but that's going more beyond hehe)

I just quickly threw these here so if they don't make sense (which will probably be the case :P) just holler for me :)