HstHtml tag doesn't need to be tightly coupled with HstRequest/HstResponse, but HstRequestContext.

Currently, HstHtml tag library checks if the request/response is HST request/response and doesn't render html if not.
However, the tag doesn't really depend on HST request/response while generating html tags, but requires those only once to get HstRequestContext which is the only part while link generation actually.
Therefore, I think it might be more useful if we let the HstHtml tag be used in non HST component templates. For example, external applications such as spring mvc might be able to use this tag library with a proper HstRequestContext and content document beans.

In summary, let's allow to use HstHtml tag library for non hst component templates. A solution might be to remove hstrequest/response checking codes and add one more line to get hstRequestContext (e.g., RequestContextProvider.get()).