New setting `requestPageContext: boolean`

[brillout]

To determine whether a pageContext.json request should be made upon client-side navigation.

So far, I can't think of a use case where this setting is needed: instead, AFAICT the following gives enough control to the user:

• New setting .once.js e.g. +data.once.js #2566

Let's try to implement this setting only as last resort. It's less than ideal (because it's coarse), but it might turn out to be an easy and practical solution.

See also:

• New setting +requestPageContextOnNavigation #2523