Add social sharing

[chrismclarke]

It would be good to display a 'share' button (possibly with social icons) that can share a link to the problem. As deep linking is now implemented simply sharing as the URL (e.g. https://mathsclub.samicharity.co.uk/en/problem-slug) should work well so that anyone who receives the message and opens will open in the app if they have it installed.

https://ionicframework.com/docs/native/social-sharing