Upgrade to Vue 2.6.X

[subdavis]

Running into https://github.com/vuejs/vue-cli/issues/1584 but there isn't anything wrong I can tell.

Part of #103, will resolve https://github.com/girder/girder_web_components/issues/100

<template v-slot> can only appear at the root level inside the receiving the component

 FAIL  tests/unit/Register.spec.js
  ● Test suite failed to run

    [vue-jest] Error: Vue template compilation failed

      2 | import { mount } from '@vue/test-utils';
      3 | import RestClient from '@/rest';
    > 4 | import Register from '@/components/Authentication/Register.vue';
        | ^
      5 | import { flushPromises, girderVue } from './utils';
      6 | 
      7 | const localVue = girderVue();

      at error (node_modules/vue-jest/lib/throw-error.js:2:9)
      at compileTemplate (node_modules/vue-jest/lib/template-compiler.js:29:5)
      at Object.module.exports [as process] (node_modules/vue-jest/lib/process.js:66:29)
      at Object.<anonymous> (tests/unit/Register.spec.js:4:1)

 FAIL  tests/unit/Breadcrumb.spec.js
  ● Test suite failed to run

    SyntaxError: Unexpected character '#' (1:165)

      at Parser.pp$4.raise (node_modules/vue-template-es2015-compiler/buble.js:2757:13)
      at Parser.pp$8.getTokenFromCode (node_modules/vue-template-es2015-compiler/buble.js:4906:8)
      at Parser.pp$8.readToken (node_modules/vue-template-es2015-compiler/buble.js:4628:15)
      at Parser.readToken (node_modules/vue-template-es2015-compiler/buble.js:6029:22)
      at Parser.pp$8.nextToken (node_modules/vue-template-es2015-compiler/buble.js:4619:15)
      at Parser.pp$8.next (node_modules/vue-template-es2015-compiler/buble.js:4576:8)
      at Parser.pp.eat (node_modules/vue-template-es2015-compiler/buble.js:577:10)
      at Parser.pp.expect (node_modules/vue-template-es2015-compiler/buble.js:641:8)
      at Parser.pp$1.parseFunctionParams (node_modules/vue-template-es2015-compiler/buble.js:1230:8)
      at Parser.pp$1.parseFunction (node_modules/vue-template-es2015-compiler/buble.js:1218:8)

[zachmullen]

The rewrite of the scoped slots documentation for 2.6 has made me understand them for the first time. This is a good change for sure.