Proper templating instead of uglish String::push_str

[gdamjan]

…not that it's that bad :)

(will see)

[gdamjan]

contenders:

• https://github.com/kaj/ructe
• https://github.com/Keats/tera
• https://maud.lambda.xyz/
• https://docs.rs/handlebars/1.0.0/handlebars/