Changing 'react-router' imports to 'react-router-dom' in AdminUI

[nathsimpson]

Solving #802

It appears the error in issue #802 was due to a version mismatch between the beta versions of 'react-router' and 'react-router-dom' we are using in the Admin UI. As 'react-router-dom' depends on 'react-router', @mitchellhamilton suggested replacing any RR import with a RRD import.

Tested with the Todo and Blog demo projects, and all seems to work.

Note: also fixed a broken import in blog demo project

[emmatown]

Could you also remove react-router from the root package.json?

[emmatown]

More context about this:

What's happening here is that react-router-dom depends on react-router@^4.4.0-beta.6 which can resolve to react-router@4.4.0-beta.7 meaning react-router won't be hoisted so react-router and react-router-dom can have different router contexts. Since react-router-dom exports all of react-router's exports, we can use react-router-dom everywhere like in this PR to solve the problem.