NEW: Allow configuration of default embed width for images and media in HTMLEditorField

This pull allows for the setting of a default embed width so that the initial width of an image can be tailored to individual website designs, rather than always being 600px.

Also included is a change to make the default alignment of images and media 'centered, on its own' as I find this more practical for most cases.

I couldn't work out how to split the two commits in to separate pull requests but hoping you can incorporate the first one at least and ignore the second one if you choose.

Thanks