Evaluate/update`PolicyManager`

[martijnc]

While working on #357 I noticed the UA based adaptive directive logic is outdated and it may need an update.

According to MDN's CSP compatibility table, most browsers have caught up to CSP Level 3, with some minor differences.

With current browser support for all CSP levels, the UA based adaptive approach may no longer be needed altogether?

[Seldaek]

I am not sure about that.. It indeed seems to be a bit too rudimentary, either we should check exact browser versions or we should probably drop this as all latest browsers are fine with Level 3. I guess the question is what's the risk? If directives are ignored it's probably fine, as long as it doesn't break older browsers. Level1 fallback stuff I would probably still keep it tho as it otherwise makes sites unusable on older browsers if you include nonce etc.