Create store/git/sys/extend/addon directory and link if it does not e…

Created by: anaqreon

…xist in all plugin repo GUI POST actions

The update repo and remove repo actions fail if install hasn't been performed because the symlink to the addon directory has not been created. This checks and create it in each POST action block. This can be done more efficiently but I wanted to issue a quick fix before many people start using it now that it's in the master branch.