Use alternate (shared) Google Drive path

### Is your feature request related to a problem? Please describe.
We would like to see if we can use floccus to share bookmarks to our group. To achieve this, we would need to store the link data file in a shared Google Drive folder.

### Describe the solution you'd like
The ability to specify a path as well as a file name for the link data file.

### Describe alternatives you've considered
Tried guessing what the path would be and tried using that in the config, but no workie.

### Additional context
N/A.

Start by locating the configuration handling for the link data file and the Google Drive storage integration. Trace how the current filename is resolved, then determine how a user-specified directory path and filename could target a shared folder. Done means the configured path works for shared Google Drive storage without breaking the existing default behavior.

Written by the indexing model from the issue text.