Remove the dependency on html5lib

Remove links to Betty's unused Twitter account

Use Babel's Locale everywhere to reduce data conversions

Translated using Weblate (Russian)

Currently translated at 100.0% (355 of 355 strings)

Translation: Betty/Betty
Translate-URL: https://hosted.weblate.org/projects/betty/betty/ru/

Reorganize the plugin API

Update the translations documentation and demo locales

Allow breadcrumbs without URLs

Fix a bug where the setuptools build script would not include the cor…

…rect Betty version

Ensure Trees styling can be overridden by themes

Translated using Weblate (Russian)

Currently translated at 45.0% (160 of 355 strings)

Translation: Betty/Betty
Translate-URL: https://hosted.weblate.org/projects/betty/betty/ru/