add signal article_generator_pretaxonomy (was: split ArticlesGenerator.generate_context tags, categories functionality)

the functionality is now in `_generate_aggregate_context`
This enables plugins (e.g. i18n_subsites) to reuse the code when
some articles are e.g. moved to drafts (e.g. HIDE_UNTRANSLATED_CONTENT)

The i18n_subsites comment is in PR state atm: https://github.com/getpelican/pelican-plugins/pull/139