Add content classes to notes

[uroybd]

This PR allows users to:

1. Set a key from which the classes should be extracted (Defaults to dg-content-classes). This reconfigurability allows users to avoid redundancy. For example, the Minimal theme comes with its own key to set classes. Users can use the same values for the digital garden too.
2. Allow users to set classes to be added to the content container from the notes. It will allow users to do many style tweaks. For example card view.

[uroybd]

Here's an example of what users can do with these classes in combination with #276

[oleeskild]

That is really smart, thanks for the PR! Merging it, and releasing it after doing some basic testing