Add seperate page for 2fa providers, allowing for a cleaner homepage

This pull request adds a page dedicated to cataloging the numerous 2fa providers. This was mentioned in [PR 192](https://github.com/jdavis/twofactorauth/pull/192) as an alternative to having an ever-expanding list of providers with a column each. I think this is more scaleable and less political (because you don't have to decide who gets their own column).

From here, I would suggest that the main site generalize Google Authenticator to "OTP" or "RFC6238" then put all options (Google Authenticator, Authy, Verisign, Toopher, Duo, Authentify, Clef, etc) into the custom column.