FSE: Allow font size, text and background color changes on site title and description blocks

[apeatling]

This functionality is already built in to Gutenberg, update these blocks to support it.

[mrfoxtalbot]

How are we planning on stripping this HTML from the site title and description once it is added to the block? I am asking this because I have already encountered this scenario in real life where a user has added HTML code inadvertently to the block and this HTML will them be rendered as plain HTML. For instance:

• Adding a line break (Shift + Enter)

• Pasting formatted text (will preserve bolds an italics).

[apeatling]

We likely won't add HTML directly, but setting style attributes that are applied to the content. TBD though.

[apeatling]

Closing this one in favor of WordPress/gutenberg#19491