Upgrade prettier to v1.15.2

What does this MR do?

Upgrades prettier to the latest version which adds Vue template support.

Prettier's opinionated formatting conflicts with (or makes redundant) the following eslint-plugin-vue rules which have been disabled:

• vue/html-closing-bracket-newline
• vue/html-closing-bracket-spacing
• vue/html-indent
• vue/max-attributes-per-line

Example changes

The diff on this MR loads slowly so here's a representative example.

VSCode warning

NOTE: The VSCode Vetur extension which adds linter/formatter support to .vue files has not been updated to recognize prettier 1.15.2. If you with to have prettier auto-format vue files on save, you'll need to add the following to your user config, to disable the formatter options in Vetur and enable them in Prettier:

{
  "vetur.format.defaultFormatter.html": "none",
  "vetur.format.defaultFormatter.css": "none",
  "vetur.format.defaultFormatter.postcss": "none",
  "vetur.format.defaultFormatter.scss": "none",
  "vetur.format.defaultFormatter.less": "none",
  "vetur.format.defaultFormatter.stylus": "none",
  "vetur.format.defaultFormatter.js": "none",
  "vetur.format.defaultFormatter.ts": "none",
  "prettier.disableLanguages": []
}