Allow users to also use their email address to login

Allow users to also use their email address to login, in addition to their username.

Their email address might be much easier to remember and/or type in, especially for non-ASCII alphabets. There is little security advantage to forcing them to type only their username when many other boards accept both email and username.

This will also save a retrieval step if they forget their username.

When implementing be sure the prompt also mentions both.