Switch libass URL to github
Move all in-repo deps to aegisub/vendor
Move submodules to aegisub/vendor
Move everything up a level since the root dir no longer has stuff
Add in-repo copy of Lua and use it rather than the system copy

Aegisub needs Lua built as C++ (for stack unwinding on errors). No one
packages it like that, so at the moment we can't actually use systme
packages for Lua.

Fixes a bunch of memory leaks on Automation errors.
Fix compilation with non-bleeding edge boost
Install CSRI and iconv headers rather than mucking about with include paths
Add some missing inter-project dependencies