Add loading state to newsletter

[maureenlholland]

Description

We used inline SVGs for loading indicator on recent newsletter updates. Use of a style tag for animation in this SVG created CSP errors (in console and on Sentry)

Bedrock: https://github.com/mozilla/bedrock/blob/main/bedrock/newsletter/templates/newsletter/includes/form.html#L114

Related

mozmeao/springfield#785

Success Criteria

• newsletter component includes user-facing loading indicator
• no CSP errors