Ported monster tank madness to lua

~~It would be nice, if someone could already have a look at this (style nits?),
although this is not completely finished yet (see todo).~~ See below.

[This](https://github.com/abcdefg30/OpenRA/commit/f5fe14701284c88ecdf66b5f8fb3f5db24a7296e#diff-da0b70bbc22b6b4af2fbe96dd5c5a8b7R19) set to true can help testing.
another suggestion: do you think it would be better to reveal these objectives as they begin to matter? Ie., have only the CrossRiver objective at the start, and then add FindOutpost once that completes, and finally add the other two?