[PP-1] HtmlResponseSubscriber and DynamicPageCacheSubscriber should have negative priorities so that vanilla response subscribers can have priority zero

Problem/Motivation

DynamicPageCacheSubscriber::onResponse() has a priority of 100.
It is documented as needing to run before HtmlResponseSubscriber::onRespond(), which has a priority of 0.

When trying to implement a response subscriber to add cache tags, it needed to be set to 101 to function.

Proposed resolution

Allow vanilla response subscribers to have a 0 priority by making DynamicPageCacheSubscriber and HtmlResponseSubscriber negative.

Remaining tasks

Pick a negative priority for each that works.

User interface changes

N/A

API changes

Not really, right?

Data model changes

N/A