Fix signature editor style to use available space

[benkahle]

This fixes the "Edit raw HTML" mode of the signature composer to grow to available space. It now matches the appearance of the default contenteditable component for plain text.

Fixes #3219 Fixes #2809

[grinich]

@emorikawa @jstejada can we merge this?

[benkahle]

@grinich Any update on this since the 2.0 release?

[dweremeichik]

@benkahle I see that this was never merged in. Since Nylas no longer supports this product, we would love for you to create a PR over at Nylas Mail Lives.