Use stable IDs for contributor and webpage BNodes to avoid false git diffs

[coolharsh55]

The RDFs contain lots of BNodes for contributors and webpages, which change whenever the output is regenerated -- causing git to show these as changes. This is not ideal as it increases the manual effort to find actual changes and also 'pollutes' the git history with changes that are not relevant.

Fix: Create local ids for each contributor and webpage e.g. using a hash or slugify function, and add it to the local graph with a prefix e.g. contributor- and web-. This will ensure the ids are consistent across git history and only the actual relevant changes are flagged.

Proposal with @bact ; see discussion in #265

[bact]

Fixed by commit 4b496c1