[election-ui-js] Use Prettier to autoformat code

[paulbrimicombe]

What?

• Use prettier to auto-format code in the election-ui-javascript project

Why?

• We have already done this in the election-api-javascript project (see [election-api-js]: Use prettier for code formatting #88)
• It is a very common pattern to use prettier (and similar tools) to automatically format code in a consistent manner. I have set it up to auto-format the project files when you run npm test.

Notes

• This is somewhat a matter of taste so feel free to disagree with this change but please do give reasons if you do!
• I promise that I've not made any changes other than:
  • Installing prettier
  • Adding the .prettierignore file
  • Adding prettier to the test script in package.json
  • Running npm test to format all the code and run the tests

[TimPerry]

LGTM