[tg bot] Clean strings that could contain special characters before adding to telegram html msg

Summary:
T2972

Depends on D13286

Add function prepareStringForTelegramHTML and unit tests. Apply to strings that could contain '<', '>', or '&' characters. Add test where these strings do contain such characters.

Test Plan:
npm test
npm run msgs

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13296