Configure available headings and formatting options

The standard TinyMCE configuration allows for specifying H1 and H2 level headings, which we have traditionally removed in Atto.

We need to decide upn the available formatting types, which we can do with the block_formats option:
https://www.tiny.cloud/docs/tinymce/6/user-formatting-options/#block_formats

We need to look at whether there are any other options we wish to remove, restrict, or otherwise configure. This may include items such as (not an exhaustive list):

• text and background colours
• font family configuration
• font size configuration