Provide sensible defaults for integrations_widgets_urls

[jaywink]

In the case that someone running a self-hosted Riot doesn't define integrations_widgets_urls at all, Riot defaults to integrations_rest_url.

Since the integrations_widgets_urls "normally" (ie in app/develop, desktop) has defaults as follows, it would make sense to default to those in the case that the widget urls are not set:

    "integrations_widgets_urls": [
        "https://scalar-staging.vector.im/api",
        "https://scalar.vector.im/api",
        "https://scalar-staging.riot.im/scalar/api"
    ],

This would ensure that widgets created with either live or staging Scalar would work even if the Riot configuration doesn't define them. It should be possible to disable all of them still by setting integrations_widgets_urls to an empty list.

This issue comes from a real world case where custom hosted Riot didn't define these urls and failed to thus render a widget created via scalar-staging.vector.im.

[turt2live]

fwiw if I get my way with the integrations API proposals then this field becomes obsolete.

[turt2live]

(also insert a comment here with my community hat about dissatisfaction for continuing to default to scalar when there are other integration managers in the world)