Drops separate PlainComponent file, makes series of small optimizations

[lourd]

• Since all PlainComponent did was render null, there's no need to make
  it class-based or a separate file. Doing a null-returning function
  inline saves some overhead
• Refactored some functions to use reduce and insert the spaces within
  the higher-order function instead of joining and trimming afterward
• Adds explicit name/reference for HelmetSideEffects and passes that to
  the Helmet wrapper
• Makes some small changes around indentation, returning earlier, and
  implicit returns from arrow functions

[cwelch5]

Thanks @lourd !