Messages while having a pending account are confusing

[charmander]

If someone misses their e-mail verification somehow, they end up in a state where

• registering the same username says the username is taken
• visiting the profile page says the username doesn’t exist
• requesting a password reset says the e-mail address isn’t registered
• registering under a different username says the e-mail address is already registered

for what looks like somewhere between 48 and 72 hours depending on how midnight UTC compares.

We can improve the last case pretty easily by explaining the situation in the e-mail and even including the activation link for the original account.