Ensure urljoins with base_url use relative paths

[lossyrob]

In several instances a urljoin is used with Fast API's base_url, where a leading / is used during the join. This works if there's no root_path set, but in the case where base_url contains a path prefix, the leading / makes the resulting join based on the host information and disregards the root_path.

Examples:
BaseLinks.root - joining to "/" erases the root_path, should just use str(self.base_url)

• CollectionLinks.parent
• ItemLinks.self - removing the leading slash will fix this.

The goal of this issue is to find all the instances where a urljoin is used with a leading slash, and joining to a relative path instead (or avoiding a join in the case where the base_url can be used directly).