Allow Grocy instance on URL path

[marcelvriend]

Fixes #195.

• Also allows a Grocy instance installed on a URL with path, e.g. https://www.example.com/grocy. You can enter the full URL including the path.
• URL input is a bit more forgiving, it now ignores trailing slashes.
• GET requests to system endpoints wrongly had a /api prefix.
• Black formatting.