Build Envoys with Holy District Project

Adds a new holy site project that provides an envoy upon completion. Cost scales by game progress from 100 to 1,500. I'd prefer it to cost less and give points towards earning envoys rather than a full envoy, but short of hacking the main dll it doesn't currently seem possible to change the influence points in a mod. I tried to make it expensive enough to not cause serious balance issues, but only play testing will tell--perhaps it should cost more or require a specific civic or tech. I'd welcome any feedback on balance or other issues.