Never update author of item on edit

[ekzyis]

Description

The combination of #1393 + #644 made it possible that the author of an item could change from anon to a user and vice versa.

This PR makes sure that during item updates, the author is never changed.

Checklist

Are your changes backwards compatible? Please answer below:

yes

On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:

8

• switched accounts and edited item with different author and made sure author doesn't change (using e04f64e before edit option disappears on switch)
• made sure hash+hmac edit only shows for anonymous items if anonymous

For frontend changes: Tested on mobile? Please answer below:

n/a

Did you introduce any new environment variables? If so, call them out explicitly here:

no