Issue 3004 - Change input name for 'username' search to avoid signin auto-fill popup

[vinkaks]

This PR is an attempt to fix #3004

REQUIRES TESTING ON SAFARI BROWSER (I don't have a iOS device.)

Safari/iOS users face a disruptive sign-in suggestion pop-up when they land on or attempt to fill in the username search form on https://tatoeba.org/en/users/all. Same issue observed but not as disruptive on other browsers.

Attempt to solve by

• Avoiding common auth forms input names like name, username etc
• making the input name unique. username is also used on login and registration pages. This could be triggering autofill if password and username are saved and autofill active
• add search to input name to highlight it is a search field

[Yorwba]

I don't have an iOS device to test this on, but the changes make sense to me and at least in Firefox the context menu stops suggesting to manage logins and allows setting a search keyword instead. Once this is deployed, mramosch can test whether this works in Safari as well.