Consolidate .gitignore files

Having these scattered about across the repository makes it hard to work
out where something is ignored, and most tools/editors that read
.gitignore files don't pay attention to those not in the repository
root.

h/static/.gitignore

-/webassets-external

h/static/scripts/.gitignore

-*.min.js
-/account.*js
-/app.*js
-/config.*js
-/hypothesis.*js

h/static/styles/.gitignore

-/*.css