Solves #564
Utterances doesn't have a loading spinner. This is fatal, because if GitHub's API doesn't respond quickly, Utterances shows nothing, making the user think the feature is broken and scroll away before the loading is complete.
I see it especially on the Smartphone, when connection isn't the best and you scroll down to the comments section.

This PR adds a loading spinner, that respects the Theme set by the user, see gifs below. The colors are taken from the Theme file variables.

Github-Light	Github-Dark
Gruvbox-Dark	Sakura-Vader