Use minijinja templates for emails

[Turbo87]

This PR refactors our email sending code to use minijinja templates instead of format!() and string concatenation. In the long term this even enables us to add dedicated templates for HTML emails too, or use template inheritance to use consistent email footers, etc.

Before:

impl Email for UserConfirmEmail<'_> {
    fn subject(&self) -> String { /* ... */ }
    fn body(&self) -> String { /* ... */ }
}

emails.send(recipient, user_confirm_email).await?;

After (with templates):

let email_message = EmailMessage::from_template("user_confirm", context! {
    user_name => user_name,
    domain => domain,
    token => token.expose_secret()
})?;

emails.send(recipient, email_message).await?;

Hello {{ user_name }}! Welcome to crates.io. Please click the
link below to verify your email address. Thank you!

https://{{ domain }}/confirm/{{ token }}

[Turbo87]

rebased to include the new email type from #11419

[eth3lbert]

A quick glance, and overall LGTM, thanks! I've also left a few nitpicks more related to unified coding style.

[Turbo87]

Thanks for the review! All of the feedback should be addressed now :)

[eth3lbert]

Nit: use chaining, as is done with others.

Suggested change

	let email = EmailMessage::from_template("new_token", context);
	let email = email.context("Failed to render email template")?;
	let result = emails.send(recipient, email).await;
	result.context("Failed to send email")
	let email = EmailMessage::from_template("new_token", context)
	.context("Failed to render email template")?;
	emails
	.send(recipient, email)
	.await
	.context("Failed to send email")

[Turbo87]

Suggested change

	let email = EmailMessage::from_template("new_token", context);
	let email = email.context("Failed to render email template")?;
	let result = emails.send(recipient, email).await;
	result.context("Failed to send email")
	let email = EmailMessage::from_template("new_token", context)
	.context("Failed to render email template")?;
	emails.send(recipient, email).await
	.context("Failed to send email")

I'd be okay with this formatting, but rustfmt seems very keen on giving .await its own line, which makes it look a bit ugly IMHO 😅

[eth3lbert]

maybe change other chained statements in other files to separate statements? I'm fine if they all follow the same style.

[eth3lbert]

These are all just nitpicks, not blockers, so feel free to merge if you want! :D