Update to 1812 on webpage

Various places on the webpage you still refer to v1806, e.g.:

https://www.openfoam.com/code/repositories.php

https://www.openfoam.com/code/build-guide.php

https://www.openfoam.com/documentation/system-requirements.php

Further, it would be convenient if the ThirdParty-xxx source could also be provided via gitlab.