Add notifications block to AllAuth templates

[ericholscher]

The allauth templates are overwriting the base templates block:

• https://github.com/readthedocs/ext-theme/blob/main/readthedocsext/theme/templates/allauth/layouts/base.html#L3 is overriding
• https://github.com/readthedocs/ext-theme/blob/main/readthedocsext/theme/templates/base.html#L63

Fixes https://github.com/readthedocs/ext-theme/issues/425