Some styles newly applied for the Woo Content blocks might
make the blocks look broken (e.g., too big font applied on the order items table).
This commit adds a check for existing emails, and for sites with existing customized the Woo email template
they are not applied unless a user saves the email in the editor.
[MAILPOET-4180]
In the Woo email customizer, we have only one font family.
The newsletter data carry different default font families for headings.
In this commit, we enforce that the renderer uses the same font family
for text and headings.
This only applies to the part of the email that is handled by the MailPoet renderer.
The part that comes from the Woo itself is handled differently.
See previous commits.
I used clone to make sure we don't accidentally save the settings.
It wouldn't probably cause any damage because we don't expose these settings to users, but to be safe.
[MAILPOET-4180]
Woo has a stylesheet that is later inlined into the email content by Emogrifier.
In this commit I replace all font family declarations by the font family set
in the Woo customizer.
[MAILPOET-4180]
