Address font scaling issue in posts

[FelberMartin]

Description

The PostItem content does not properly reflect users changes the font scaling.

1. The post content rendered in the markdown textfield does not get affected by the custom font scaling
2. The post header has a fixed size and with bigger font sizes the post author text gets cut off

** Expected Behaviour **
The post content should scale with the fontScaling and the post header should not be cut off.

Screenshots