Use a templating engine to generate the workspace invitation emails #260
Description
Currently we're using formatted strings to generate the body of email notifications (e.g. to invite someone to the workspace). If we want to localize those notifications (which we do, issue coming soon), it'll be easier to use a templating engine and maintain localized variants.
For this, we could use Tera (https://crates.io/crates/tera) which is lightweight and effective. In addition it is developed by a trusted developer, the creator of the validator crate (which we will soon be using).