Phabricator: https://phabricator.wikimedia.org/T376300

Notes

This PR fixes some issues with the sharing view:

1. Facebook and Instagram sharing is fixed. We're only sharing the image - it seems like having both text and image caused issues.
2. Fixed cropping issues on the last donate slide
3. Fixed markdown display in share view subtitle.

Note: I have noticed the subtitle truncates. I'm not sure if this was introduced by this PR or not, but Product has said this is ok for V1.

Test Steps

1. Check to see if you can share to Facebook and Instagram (can lean on QA for this).
2. Check that last donate slide sharing looks good (gradient crop and learn more url).