[Feature]: allow adding optional id to the body tag

[nhi-vanye]

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Description

I'm trying to override some formantic-ui CSS and stackoverflow suggestions (https://stackoverflow.com/questions/55005996/overriding-styles-in-semantic-ui-react) are to add a id tag close to top of the hierarchy.

Problem

Easier overriding of CSS

Proposed Solution

Add optional parameter to SemanticPage body_id

Alternatives Considered

Adding additional divs, but didn't produce hierarchy as nabber (formantic.plus) has its own idea of document structure and I want my CSS to be across both nav bar and main content...

[nhi-vanye]

I an id attribute was always added to body (i.e. default = "app"), even if the SemanticPage call was hidden inside navbar_page I could still override it with htmltools::addTagsAttributes while I work on PR for an improved navbar