This is about the same amount of code than the previous approach, but
has the following advantages:
* Uses Roda's render plugin, so templates are cached and optimized
into compiled methods. The previous approach created 5 separate
Tilt::ErubiTemplate objects for every email rendered. The new
approach does not create any Tilt::ErubiTemplate objects after
the first email.
* Uses part instead of render for simpler rendering with locals.
* Moves EmailRenderer to separate file, so that reloading works
correctly.
* Skips the rendering of the email stylesheet, since it does not
contain any ERB code. Instead, the file is included without
rendering.
* Uses fixed locals for the email templates, so providing an
invalid local will result in an error.
* Removes unnecessary empty `<style>` tag in email layout.
The only spec change is adding an email to one of the invoice
specs. I'm not sure why this didn't fail before, but the
mail library complains if it tries to deliver a email with
no recipients.
2b58602ead