pelican-bootstrap3: add support of `LINKS_WIDGET_NAME` and `SOCIAL_WIDGET_NAME`

Feature request for pelican-bootstrap3 to allow changing the widget names via  [`LINKS_WIDGET_NAME`](https://docs.getpelican.com/en/4.7.1/settings.html#LINKS_WIDGET_NAME) and [`SOCIAL_WIDGET_NAME`](https://docs.getpelican.com/en/4.7.1/settings.html#SOCIAL_WIDGET_NAME) as introduced in [Pelican 3.7.0](https://docs.getpelican.com/en/latest/changelog.html#id37).