Makefile: make explicit that node_modules is dependency of base

[matthew-white]

This PR adds node_modules to the list of dependencies of the base task in the Makefile. node_modules was already a dependency, because base depends on node_version, and node_version depends on node_modules. However, I think it'd be clearer for us to include node_modules in the list explicitly. node_modules is conceptually distinct from node_version: even if we got rid of the node_version check, base would still need to run node_modules. In other words, node_modules is not just a sub-dependency of node_version.

We do something similar with node_version and the migrations task. base runs migrations, and migrations runs node_version, yet we still include node_version in the list explicitly.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced