Add password strength feedback UI

[nbarbettini]

I built the password strength feedback UI stuff from the mockups. The rules are pulled from the passwordPolicy model of the /register endpoint.

@alexdahl - the icons you sent me on Slack actually don't match up with the Invision mockups. Not a big deal, this isn't super high prio. :)

I'm not sure if it makes sense to hide this completely before the user starts typing, and have it slide out?

Here's what it looks like:

Closes #148

[alexdahl]

@nbarbettini, oh sorry totally still had my head in the reset password flow.

Proper assets: password-icons.zip

Yeah, sliding out on first focus would be nice. I think good states would be:

• no focus, no errors: password requirements hidden.
• field has focus, no content: requirements shown, each condition's text is black.
• field has focus, some content (user is typing): requirements shown, met conditions turn gray and gain a check icon.
• field blurs or form submission attempted, not all conditions met: requirements shown, all unmet conditions turn red and gain an error icon.
• field regains focus with errors (user is typing): requirements shown, unmet conditions stay red and keep icon.
• field has focus, all conditions met: requirements shown, all conditions are gray with check icons.
• (if field blurs once filled out without errors, we're back at the first state and the requirements can be hidden)

But very aware this is a lot, and I bet I've missed something. So certainly nice-to-haves.