Spec out API for apps to set further CSP restrictions.

[zenhack]

Per #3410

Nothing is actually implemented yet; I'd like feedback on the design.

[kentonv]

Realistically what kinds of restrictions do we think apps are going to want to apply to themselves? I can see the use case for disallowing inline and eval, but beyond that, is there a strong use case for other restrictions? It seems like only those two bits are interesting -- I'm not sure what legitimate security concerns are solved by restricting the others (beyond what Sandstorm already intends to do by default).

If apps are only really likely to use those two bits then I'd say we should define two flags and leave the rest out. Otherwise I think the logic to merge the app's CSP with Sandstorm's policy is going to be sort of complicated without adding that much value.

[zenhack]

@mnutt, have an opinion (since IIRC you suggested this on IRC)?

I don't feel that strongly; unless others see a reason to keep the full set I'm fine removing it -- it'll be easy to add later if we want it.

[ocdtrekkie]

Whatever magical algorithm decides who I can assign to what has said I can't assign @mnutt to this.