Support matrix servers in a subdirectory and automatically append "/"

Previously, the user-provided URL was join'ed with an absolute path (e.g. "/_matrix/..."). Thus, the path was replaced with the given one. In other words, fractal assumed that matrix servers are always available at "/".

This commit appends a relative path, so that matrix servers in subdirectories are supported as well.

Additionally, since a trailing slash is important to not overwrite the last path component, it should be automatically appended.