Retirement: BEM fixes

[anselmbradford]

Additional cleanup followup to #8274

Removals

• Removes unused thank-you class.
• Remves unused helpful-btn class.

Changes

• Update lifestyle-btn__active to lifestyle-btn--active modifier.
• Update notification-input__warning to notification-input-warning block.

How to test this PR

1. http://localhost:8000/consumer-tools/retirement/before-you-claim/ should show golden border around age inputs when date is incorrect (put in letters, for example).

[wpears]

Should this and the others like it be --warning?

[anselmbradford]

@wpears No, I was second-guessing this too, but this isn't a m-notification class, therefore it's not part of the m-notification block and is its own block, notification-input-warning.

The existing markup looks like this:

It is neither an element of m-notification, nor a modifier on another class. It should probably be re-written as a field-level alert, but that's a separate issue.