feat: issue/pull image file uploads #62

Add a way to attach files, specifically images, to issues and pulls (preferably also for repository markdown files).

You could use cloudflare R2 as a bucket storage. If you think that this would be too expensive on tangled's side - maybe give as an option to add our own S3 compatible file hosts to the settings to which we would then upload files to via the UI. With this everyone would just pay (or be in the free tier) for what ever they upload 'to' (via) tangled.