AO3-7393 AO3-7395 Fix blank/invalid pseud editing behavior

[techlingyi]

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Issue

https://otwarchive.atlassian.net/browse/AO3-7393 - Trying to change a pseud's name to an invalid one makes it so you can't update the pseud again

https://otwarchive.atlassian.net/browse/AO3-7395 - Trying to change a pseud name to an empty one results in error 500

Purpose

Reloads the pseud's original ID if the new pseud is invalid in some way, so that the Show and Update buttons on the edit pseud form are no longer broken upon entering an invalid pseud. This also fixes the 500 error that appears with a blank pseud. An instance variable is used to keep the user's attempted pseud name in the text box (because it would revert back to the original otherwise).

Credit

Ling-Yi (any pronouns)

[techlingyi]

Wow, I haven't seen that Rubocop error before! (A fluke, maybe? If anyone could rerun it that would be nice — it only flagged one error when I ran it the first time, on an identical branch I closed because the commit history was getting confusing.)

Edit: Thank you! :D