Implement new folder structure

[cjcenizal]

As noted in this Architectural Problems comment by @stacey-gammon, our folder structure makes it difficult to discover files and relationships:

Currently it is difficult to quickly know where to look for certain files, or logic. For example, we have:

• ui/public/directives/saved_object_finder.js
• ui/public/saved_objects/saved_object_registry.js (the only file in this folder)
• ui/public/courier/saved_object.js

The angular style guide (see #9049) has some suggestions around folder and file structure

Steps

• Add Angular Styleguide docs (Implement AngularJS style guide #9049)
• Decide on a target folder structure, justified by some kind of paradigm (e.g. separation of business logic and "dumb UI components" into services and components folders).
• Test it out by migrating a couple files into the new structure.
• Migrate files on an ad-hoc basis as part of every PR.

[cjcenizal]

Per discussion, let's prove out this directory structure in other, small projects first, e.g. Watcher UI.

When we begin implementing the new structure in ui/public, let's also be wary of gotchas/special cases which don't have a clear place in the targeted folder structure.

[cjcenizal]

So far, the EUIfication process has given us the opportunity to reorganize files in a clearer way, e.g. EUIfying index patterns. If this keeps up then this issue is moot.