feat: allow users to input instance URL without a scheme

kmasto allowed users to enter instance URL without scheme, but this created invalid URLs to get the token from. If user had forgotten to enter the scheme, it created a domain-less URL. In the case of a http:// url it would not receive any data on the POST request to the instance to create a client ID. This MR attempts to upgrade http connections to https and automatically fix the instance URL using QUrl::fromUserInput()