Improve two-step log in flow

[isaackeyet]

UX rough patch: if you have two-step authentication enabled for your WordPress.com blog, the process to add it in is all but smooth.

1.Sign into a WordPress.com account with two-step authentication enabled
2. A popup tells you you can't log in because you have two-step enabled, and that you need to generate an app specific password (but this seems to display in the Select Blogs screen on iPad so if you dismiss it you end up with an "empty" list and have to tap Back)
4. Go Back
5. Figure out what an app-specific password is
6. Generate app-specific password on desktop
7. Enter in all your details again

The expected process would most likely look like this instead:
(0.) A check on username is performed when entered to see if they're WPcom and if they have two-step enabled. If both are true, the description of the password field changes and there's info text about two-step below it, including a link to an FAQ entry and/or a WordPress.com support page

1. Enter your credentials
2. Learn what an app-specific password is
3. Generate app-specific password on desktop
4. Enter it into the field, log in

If we can't check for two-step like this I think we can at least do some work on the error message -- maybe use the new error messages, exchange the field description to also include a link to an FAQ entry or similar.

[sendhil]

Closing as dupe of #269