Update Stylelint configuration to forbid CSS !important

[aduth]

🛠 Summary of changes

Updates @18f/identity-stylelint-config shared Stylelint configuration to enable the declaration-no-important rule.

Related changelog entry, from proposed code:

Breaking Changes

[...]

• The declaration-no-important rule is now enabled, which disallows !important in stylesheets.
  • !important is a sledgehammer solution which often causes more problems than it helps, and usually stems from misunderstandings of CSS specificity. See related "Best practices" MDN documentation.

As part of this, the changes remove one instance of !important currently used in email stylesheets. These styles appear to be redundant, as they're already applied via upstream Foundation email styles (source).

📜 Testing Plan

Verify no regressions in the padding of email templates at small viewports.

Compare:

• Before: https://idp.dev.identitysandbox.gov/rails/mailers/user_mailer/add_email
• After: http://localhost:3000/rails/mailers/user_mailer/add_email