Simplify session timeout modal presenter

[aduth]

Extracted from #7966

🛠 Summary of changes

Simplifies the behavior of the session timeout modal presenter:

• Flatten separate and largely repeated classes FullySignedInModalPresenter and PartiallySignedInModalPresenter to a single SessionTimeoutModalPresenter
  • Name presenter such that it's obvious its purpose in handling session timeout behaviors
• Move view rendering responsibilities out of presenter class and into view partial, limiting scope of the presenter class to handle differences between "fully" and "partially" signed-in users
• Use standard design system buttons implemented through ButtonComponent
• Avoid unclear abbreviation "sr" in favor of live_region_ qualifier describing its context in a ARIA live region, which notably can benefit more than just screen readers
• Avoid setting initial expiration time in modal content, expecting that this will be assigned by the session timeout JavaScript [1] [2]

📜 Testing Plan

Confirm no regressions in the behavior of the session timeout modal, including specifically:

• Differences between "partially" and "fully" signed in (i.e. before and after MFA authenticated)
• "Stay signed in" and "Sign out" buttons work as expected
• Modal text counts down

It's easier to test by setting a very low session timeout in local config/application.yml:

session_timeout_in_minutes: 1

[zachmargolis]

LGTM