politeiad: Cleanup middleware.

[lukebp]

This diff makes the following changes to politeiad:

• Cleans up the existing middleware. Instead of registering middleware
  on individual routes, the middleware is now registered on the router
  itself.
• Adds new middleware for a maximum request body size.
• Adds tests to verify the request body size middleware.
• Adds a config options to make the maximum request body size
  configurable.

[chappjc]

Good stuff. I just noticed that setupBackendGit doesn't use it's input arg, but that's tangential to this work.

[lukebp]

I just noticed that setupBackendGit doesn't use it's input arg, but that's tangential to this work.

whoops, good catch. I'll fix that.