[1.0] Loading indicator for "Submit comment"

[Nutomic]

When writing a comment and clicking the "Post" button, it sometimes takes half a second or more for any visual feedback. Then the normal instinct is to click again, which results in the comment being posted twice.

To resolve this the button should be disabled and show a loading indicator. 0.19 has this but somehow it was removed in 1.0

[dessalines]

I'll take a look at this sometime soon.