Fix double-absolute SpaceDock URLs

[HebaruSan]

Problem

After KSP-SpaceDock/SpaceDock#388, lots and lots of SpaceDock mods had their resources.x_screenshot property updated to start with https://spacedock.info/https://spacedock.info/:

KSP-CKAN/CKAN-meta@2de283f...bea11c1

Cause

The SpaceDock mod API used to return background as a relative URL, and now it's absolute. Netkan has code to convert it from relative to absolute unconditionally, so now it's double-absolute.

Changes

• The JsonConvertFromRelativeSdUri converter is removed from SpacedockMod.background
• The JsonConvertFromRelativeSdUri class is moved to its own file
• SpaceDockApi.ExpandPath is moved into JsonConvertFromRelativeSdUri and made private
• SpacedockMod.GetPageUrl is refactored to account for ExpandPath now being private
• Tests.SDMod.SD_URL_encode_816 is refactored to test URL processing as part of full parsing of mod JSON rather than just the now-private ExpandPath

Now the absolute URL from SpaceDock will be used as-is, so most of those commits should revert after merge.

[DasSkelett]

Let's get this fixed again!