Allow the Contribution Tool URL to be configurable

[alessandrozago]

The URL for the contribution tool in the reporter module is written directly in the code. It would be nice to have it configurable in case a custom contribution tool is installed. This is from src/reporter/index.js:8 const CONTRIBUTION_TOOL_URL = 'https://contribute.opentermsarchive.org/en/service';

This URL is used to create the link for "Edit the declaration" when opening an issue on the declarations repository.

Available for clarifications.