Loading Spinner

Solves https://github.com/utterance/utterances/issues/564 Utterances doesn't have a loading spinner. This is fatal, if GitHub's API doesn't respond quickly, as Utterances shows *nothing*, making the user 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](https://github.com/utterance/utterances/pull/677) | |--------|-------| |  |  |