Do not base "Status" of gravity run on HTTP response but actual file content

[yubiuser]

Follow up FR as requested by @DanSchaper.

Pihole should consider adlist content as well to decide if adlist was updated

I've noticed that (most of) my adlists get the status Status: Retrieval successful instead of No changes detected when running pihole -g, even when running it twice within a minute. Looking at the code, I see that the ''Status' is based on HTTP response. The code even states that some servers don't provide the necessary header. As pihole will save all downloaded lists locally, I request to additionally compare old local lists and (new) downloaded to decide if "Status" should be "changed" or "n…