Version check

Rebased version of https://github.com/Elgg/Elgg/pull/5293

@ewinslow Following your remarks from https://github.com/Elgg/Elgg/pull/5293

The main reason for keeping elgg.org fallback was case with missing openssl and thus https capability required to call Github. It's implementatnion is hacky though and I would consider dropping it and just require openssl for version check to work.

I'm not sure if I can use any specific part of manifest to get repository URL (`repository` element?). It deserves separate page IMHO or direct plugins page integration.