Raise error when view missing title content block

[aduth]

🛠 Summary of changes

Updates base template to enforce the expectation that a controller view will assign a content_for(:title), to satisfy descriptive purpose requirements of WCAG SC 2.4.2 Page Titled, something we already partially enforce through accessibility matchers in specs

📜 Testing Plan

Build should pass.

Also verify that an error would occur when controller view doesn't include title, such as removing this line from the sign-in page:

identity-idp/app/views/devise/sessions/new.html.erb

Line 1 in 43e1303

<% title t('titles.visitors.index') %>

[aduth]

I think I've tackled the last of the missing titles in 2f7ff9d . For extra assurance, I also manually stepped through every file in app/views to double-check that every controller action view includes a <% title %> statement, as well as some of our base-layout-rendering partials.

I'll need to get a couple translations for the missing titles before this is fully ready.