Social Share buttons: add UTM campaign codes to shared URLs

[paulhyland]

This PR adds UTM campaign code query string parameters to URLs shared by social share button links. These parameters follow CFPB guidelines for such campaign codes.

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Additions

• facebook_campaign etc: URL encoded query strings for URLs shared via Facebook, X, LinkedIn, and email.

  note: A value for linkedin_campaign is included here so that the proper query string values are documented, in case LinkedIn restores support for such query strings in shared URLs.

Removals

• redirect_uri query string parameter in the Facebook share_url (it is no longer needed).

Changes

• added the new query string parameter strings to the share_url values for each social share button

  note: linkedin_campaign is added to the LinkedIn share_url string, even though it doesn't appear to be supported currently - just in case support for such query strings returns. It does no harm, but rather is simply ignored.

How to test this PR

1. Go to a page on CF.gov that includes share buttons; these include blog or newsroom posts and some language or report pages.

2. Click on the Facebook, X (Twitter), and Email share buttons, and confirm that the proper campaign code query strings are added to the links shared via each of these methods.

   Note: If you're testing using localhost:8000, Facebook share won't work, because this platform apparently checks to make sure that the URL you are sharing is reachable.

3. Click on the LinkedIn share button and make sure that it still works, provides the same share dialog as the live site.

Screenshots

Before	After

Notes and todos

• If testing using localhost:8000, please loop back to test the share buttons again on the live site and ensure that they are working as expected (see: How to test this PR above).

Checklist

• PR has an informative and human-readable title
  • PR titles are used to generate the change log in releases; good ones make that easier to scan.
  • Consider prefixing, e.g., "Mega Menu: fix layout bug", or "Docs: Update Docker installation instructions".
• Changes are limited to a single goal (no scope creep)
• Future todos are captured in comments and/or tickets

Browser testing

Check the current browser support cutoff list](https://cfpb.github.io/consumerfinance.gov/browser-support#current-browser-support-metrics) for browsers that are advisable
to prioritize for testing.

Accessibility

• Keyboard friendly (navigable with tab, space, enter, arrow keys, etc.)
• Screen reader friendly
• Does not introduce new errors or warnings in WAVE

Other

• Is useable without CSS
• Is useable without JS
• Does not introduce new lint warnings
• Flexible from small to large screens

[anselmbradford]

LGTM 👍