feature(client): Prevent users from seeing confirmation pages if they have not initiated the process.

* For /confirm, redirect to /signup
* For /confirm_reset_password, redirect to /reset_password

fixes #847 

@ryanfeeley and @johngruen - I need help from the master wordsmiths! "Invalid token" isn't really a helpful message, and we use it in a couple of places.

If a user visits one of the confirmation screens without initiating /signup or /reset_password, I send them back to the /signup or /reset_password. I'd like to print a helpful message, right now it just says "invalid token" to be consistent with an already existing error message. HALP!

Something like `You must sign up first` or `You must start a password reset first`?