Add basic font families. For now, we use know-how from the current editor
and we use the same list of fonts as were used in the current editor
[MAILPOET-5740]
We don't reset font family on any level, so there is no need to
bubble the setting using a preprocessor and render the inline styles
explicitly in every block.
In this commit, I change how font-family settings are distributed/rendered
in the email renderer. In the new approach, we rely on class names defining font-family
and a generated CSS sheet with font-family definitions.
We apply the font-family CSS by inlining CSS rules for families in the later phase of
rendering after all individual blocks are processed.
[MAILPOET-5740]
