Limit number of characters that can be entered in TOTP field

[mwchase]

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There are several issues here, not all of them with GitHub, but I'm focusing on the thing that seems easiest. Here is what happened:

• I went to log into GitHub on my phone's web browser.
• I never got a working verification push from the GitHub app.
• So I tried to get a verification code from my TOTP app.
• The code didn't copy the first time.
• It turns out I had several hundred characters of text in my clipboard, and all of that went into the verification field, even though it's only supposed to contain six digits.

Please consider setting a maximum number of characters in the TOTP field, so that incorrect pastes get truncated to a more tractable length.