Ensure SPA HTML files get loaded with UTF-8 encoding

Fixes issues with wrong encoding of the "Loading Funkwhale…" text in index.html. If loaded via the backend the index.html got read with latin1 encoding, resulting in the ellipsis character being garbled as shown in the screenshot.

The reason is for how the requests module handles the encoding and tries to read it from the HTTP headers, see https://requests.readthedocs.io/en/latest/user/advanced/#encodings

Explicitly set response.encoding to enforce proper encoding is used for reading.