templates: pass ui classes through macro parameters

[kpsherva]

• closes Featured communities: CSS issues CERNDocumentServer/cds-rdm#324
• change is backwards compatible

Attention (for CDS-RDM)

1. we need to backport it to v3

[jrcastro2]

LGTM! Just a very minor comment.

[jrcastro2]

Very minor: Since there is no more styling we could simplify this to something like (styles could be a conf variable as well, if someone would like to customize it):

def style_category(category):
    """Return predefined Semantic-UI classes for each banner category."""
    styles = {
        "warning": "warning",
        "other": "grey",
    }
    return styles.get(category, "info")

[slint]

Agree, and I would also avoid the .format(...), and just go with early returns, we're not doing anything extra with the string tempalte.

[kpsherva]

I think we could in general rethink how this works and it is a bigger subject of mixing the backend with UI params. Please merge if you consider this good enough