Notify users of input validation with aria-live

[shainanigans]

Describe the bug
For the input validation, there should be an aria-live="assertive" on the validation message in cases where the message is added to the DOM/view. (There’s an example of this here from Deque.) With this attribute, screen readers will read this announcement to users when it appears. There is no mention of this in the Stacks documentation.

To Reproduce
Steps to reproduce the behavior:

1. In Pickles (or any other SO4T instance) go to Questions, open the filter, and save a custom filter.
2. Use a screen reader of your choosing to observe what happens when you change a filter and try to save another custom filter with the same name.
3. Note that the validation message is not read out.

Expected behavior
Include a note in Inputs > Validation states about using aria-live.