Refactor admin.js for readability and modernization

[patrickmj]

Old admin.js needs to be refactored so all the individual pieces are coherent and findable.

It's in Backbone, but that will eventually be removed. To maintain backward compatibility during our upgrades, it will need to remain a Backbone app. However, within each method there is plenty of work to do. Functionality should not change or be visible to end users, but the code quality will be deeply improved.

You will see:

• long if...then...else chains that could be consolidated
• semantically meaningless variables and names, e.g. if tab == 1, where 1 doesn't really say what it is
• 7 + year old javascript
• opportunities for further modularization
• and more

Focus on the following in admin.js, and make changes anywhere else necessary to make them work as needed.

• refactor admin.js
• refactor getDRSItems #261
• refactor selectItem #262
• refactor insertShortcode #263