Summary:
Grunt is hardcoded to use paths relative to wherever the Gruntfile is
located. Unfortunately it also expects the grunt packages to be siblings
of that gruntfile. We can get around this by changing the relative base
path, but then the cwd is different for each tasks. This is okay as long
as we use absolute paths for various files in each of our tasks. This
updates our grunt tasks to use absolute paths
Test Plan: `npm run build-client`
Reviewers: spang, halla, jerm, juan
Reviewed By: juan
Differential Revision: https://phab.nylas.com/D3987