Use `ejs` to render emails

[moshfeu]

Using ejs would allow us make the templates more sophisticated. For instance to render a whole section based on a condition etc.

From now on ejs.render replaces injectData - https://github.com/Coding-Coach/find-a-mentor-api/blob/8375917a16ab5670b65bcce5f80da063e2976fbb/src/modules/email/email.service.ts#L50