Prepare Admin UI for migrating to Angular.io

AngularJS is soon end of life, it enters LTS in july 2018, whereupon it will only receive fixes to serious bugs. Solr uses AngularJS 1.3 (the latest AngularJS will be 1.7).

This issue is not for upgrading to Angular5/6, but to start preparing the existing UI for easier migration later on. See https://angular.io/guide/upgrade.

This JIRA will likely get multiple sub tasks such as

• Change to Folders-by-Feature Structure, i.e. mix html, css, js in a folder based on feature
• Use a Module Loader like Webpack
• Use Component Directives (requires first move from AngularJS 1.3 to 1.5)

The rationale for this lira is recognising how central the Admin UI is to Solr, not letting it rot on top of a dying framework. Better to start moving step by step and perhaps write all new views in Angular 5, than to fall further and further behind.

This effort of course assumes that Angular.io is the path we want to go, and not React, VueJS or some other new kid on the block