Use proper templating for emails

[Nutomic]

Right now the HTML for emails is just hardcoded, which is very ugly and also means that translations are not really possible. We should extract this into template files or something along those lines.

Example, you can find others by grepping for send_email.

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[dessalines]

Yup this is a big one I left for way later.

It should probably come after #440 , that way the db could read which languages a user can read, and send emails in a preferred language.

[Nutomic]

arewewebyet.org lists some l18n related crates. Looks to me like fluent would be best for our use case. Also this doesnt depend on #440 at all, we already have a language setting in the user profile which can be used for emails.

Edit: cargo-i18n might be better, because it supports common file formats.