Run Flow for each renderer separately (#12846)

* Generate Flow config on install

We'll need to do pre-renderer Flow passes with different configs.
This is the first step to get it working. We only want the original version checked in.

* Create multiple Flow configs from a template

* Run Flow per renderer

* Lint

* Revert the environment consolidation

I thought this would be a bit cleaner at first because we now have non-environment files in this directory.
But Sebastian is changing these files at the same time so I want to avoid conflicts and keep the PR more tightly scoped. Undo.

* Misc

.gitignore

@@ -1,5 +1,6 @@
 .DS_STORE
 node_modules
+scripts/flow/*/.flowconfig
 *~
 *.pyc
 .grunt
@@ -21,4 +22,4 @@ chrome-user-data
 *.iml
 .vscode
 *.swp
-*.swo
+*.swo