[web] Remove extraneous .js suffix at end of some imports

Summary:
Noticed that my editor includes the .js suffix for JS files when it "auto-imports" them.

I'll look into an ESLint thing or something, but just figured I'd cut them for now.

Test Plan: flow

Reviewers: def-au1t, palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3706