ISBAT Copy text to a new page/post and have the custom font face be copied over

### Describe the bug:

When copying text that has a custom font face added (through the toolbar settings) and pasting into a new page or post - the custom font face does not come across - even though the settings are still in place.

I suspect that the inclusion of the font files isn't triggered if the inline font code is just pasted in - but not sure.

### To reproduce:

1. Add content to a page - and style some text (in headings or pargraphs) with a custom font family (using the Coblocks font toolbar setting).
2. Copy the blocks containing the text with custom font
3. Create a new page/post and paste in the content
4. Publish/Preview the page and view it.

### Screenshots

Source Content (Oswald typeface shown):

Pasted Content (Oswald typeface set, but not displayed):

CSS class included - but font not styled on front end:

### Expected behavior:
I expect the pasted content should display formatted in the same way as the source content

### Isolating the problem:

- [x] This bug happens with no other plugins activated
- [x] This bug happens with a default WordPress theme active
- [x] This bug happens **without** the Gutenberg plugin active
- [x] I can reproduce this bug consistently using the steps above

### WordPress version:

### Gutenberg version: