chore: update the codebase (e.g. directory layout, file naming etc)

[gkalpak]

Once/If #341 is merged, it would be a good idea to update the code organization to adhere to latest best practices™ and bring the seed closer to the tutorial (see angular/angular-phonecat#326). E.g.:

• Rename components/ to core/ or common/.
  Now that "components" means something different, it can be confusing.
• Rename _test.js files to .spec.js.
• Rename module definitions files to .module.js.
• Update karma.conf.js to account for the above changes.
• Optionally, use components when appropriate.

[gkalpak]

Some of the points above will be addressed in #444.
We won't be making more changes now that AngularJS is in LTS mode.

[petebacondarwin]

Core makes it sound like it is part of AngularJS no? I could live with common or shared

[gkalpak]

I personally prefer shared as well, but I tried to lousely follow the Angular style guide. It suggests having a shared directory/module and a core directory/module.
I felt that the type of things we had in components/ was better suited for core/ (but happy to move to shared/ instead).

[petebacondarwin]

I am not going to fight over this one