Feature: Add Font size as a property of text formatting

[horacioh]

Problem

Add the hability to change the font size of text in documents using the formatting toolbar

based on #57

[ericvicenti]

I feel like the font size should be consistent between documents for a nicer reading experience (so the font size doesn't jump around based on the document authors)

If the reader wants larger text because they can't see well, the reader should have the option.

When the author wants to use larger text, they can use a heading

So I don't agree that font size belongs in the formatting toolbar

[SalvaZaraes]

In some cases having a smaller font is useful to denote, for example, a quote.
Another option would be to add a Block called "Quote/Citation block" that automatically made text smaller, cursive and bold; adding also the option to embed to source of that quote in the block, so when someone click in da block, it directly jumps to the source.