Allow "indicate if page is saved" with non-https url-s

[poisonborz]

There are a number of cases where http:// is actually secure, eg. VPN-s or secured local networks. It makes no sense to flat-out restrict this. There should be a big fat warning and a sentence of explanation, but the only users needing this are the ones who self-host, so not security-illiterate people.

[joshua-beck-0908]

Yeah. There are many reasons the protocol can be misleading.
A warning rather than a straight-up block would be nicer for these corner cases.

As a workaround, you can always:

1. Save the JSON file.
2. Edit the file and change "AllowExistCheck" to true.
3. Load the JSON file.