Revise Summernote toolbar

[joemull]

The Summernote toolbar has been trimmed down a bit too much, and it's worth expanding it back to support a few key semantic HTML elements, like hr. There may be others as well.

We will just need to check elements the toolbar adds against the bleach settings, to make sure users can't create elements or attributes that will be removed.

https://github.com/BirkbeckCTP/janeway/blob/6dc0a2da77f3b563d6284b78ba6cf84f10294e1a/src/core/janeway_global_settings.py#L339-L351