[2.x] Fix the AJAX form validation

Problem/Motivation

In both 2.x and 8.x-1.x, we have the ability to apply our validations using AJAX. This ability could be enabled through the "ajax_validation" option.
Unfortunately, this AJAX implementation seems to be broken / not properly implemented.
For now, the "ajax_validation" config is set to FALSE, and isn't manually adjustable anymore in 2.x until this issue is fixed.

Note, that it might make sense to include the fixes in 📌 [2.x] Define user name validations as validation constraints Active , but of course this can also be fixed separately (but it might spare us some time).

Steps to reproduce

Proposed resolution

Fix the AJAX form validation,

Remaining tasks

User interface changes

API changes

Data model changes