Allow multiple pages/sections declarations per rule in templates.json files

[fdelaneau]

Instead of:

"factsheets": {
    "stylesheets": [
        {
            "url": "/css/page.css"
        },
        {
            "url": "/css/factsheets.css"
        }
    ]
},
"factsheetsentry": {
    "stylesheets": [
        {
            "url": "/css/page.css"
        },
        {
            "url": "/css/factsheets.css"
        }
    ],
    "javascripts": [
        "/js/factsheet-payroll.js"
    ]
}

Allow to write:

"factsheets, factsheetsentry": {
    "stylesheets": [
        {
            "url": "/css/page.css"
        },
        {
            "url": "/css/factsheets.css"
        }
    ]
},
"factsheetsentry": {
        "javascripts": [
            "/js/factsheet-payroll.js"
        ]
    }
}

Note that this feature will somewhat be redundant with the Swig like inheritance #7 as you could then write:

"factsheets": {
    "stylesheets": [
        {
            "url": "/css/page.css"
        },
        {
            "url": "/css/factsheets.css"
        }
    ]
},
"factsheetsentry": {
    "stylesheets": [
        {
            "inherit": "factsheets"
        }
    ]
    "javascripts": [
        "/js/factsheet-payroll.js"
    ]
}