Refactor font family rendering using CSS inlining

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]

mailpoet/lib/EmailEditor/Engine/Renderer/Renderer.php

@@ -49,6 +49,7 @@ class Renderer {
     $renderedBody = $this->renderBlocks($parsedBlocks);
 
     $styles = (string)file_get_contents(dirname(__FILE__) . '/' . self::TEMPLATE_STYLES_FILE);
+    $styles .= $this->settingsController->getStylesheetForRendering();
     $styles = apply_filters('mailpoet_email_renderer_styles', $styles, $post);
 
     $template = (string)file_get_contents(dirname(__FILE__) . '/' . self::TEMPLATE_FILE);