Provide MaterialUI via CDN

[GoelBiju]

Description:
This issue has been branched from #101 .

With SciGateway updated to MaterialUI v4 we can provide the library via CDN (ADR 7).

We need to investigate if there is any benefit in providing the whole library once as opposed to making each plugin import only the component it needs to (though this may lead to multiple imports of the same component). We will also need to look into how the bundle size compares with each method; this requires the packages/plugins to be in a position where they are nearly complete in order to compare bundle sizes - see minimising bundle size.

Acceptance criteria:

• Investigate which method results in less code being downloaded to the client; external dependency vs tree-shaking.