Provide different options for the footer

[rowanc1]

There should be a few different options for the footer:

• Full Curvenote footer (with legal terms, etc), this is black, full
• "Supported by" thin footer, blue with social links, support
• "Theme designed by Curvenote" thin footer, light-grey (dark:bg-stone-900), very subtle and should not catch the eye. design

The template options should be able to switch between these with an enum: curvenote-footer: full. The default footer should be design.

[stevejpurves]

design:

support:

full:

[stevejpurves]

@rowanc1 any changes to styling / layout here? once we are happy i'll move footers back to @curvenote/footers before releasing

[rowanc1]

I think just centering the "designed-by" footer.