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Separate session/helpers bundles, tweak bundling

Tweak the way we organize our asset bundles for fewer output files,
straightforward names, and no conflict of dev and prod output files.

- Break session and helpers into their own bundles.

- Bundle vendor libs up where they are not common to more than one
  optional component. For example, angular-resource is needed by auth
  and the app, but angular-bootstrap is only needed by helpers since
  that is where we redefine tabbable and create tab-reveal (in a new
  helpers/ui-helpers.coffee file). Another example is angular-resource,
  which is only used by the session bundle.

- Inline icomoon.css.

- Rename some outputs:
  - hypothesis-app.{css,js} -> app.{css,js}
    This is the main site application.
  - hypothesis-inject.{css,js} -> hypothesis.{css,js}
    This is the inject script.

Close #1555
Related to webplatform/notes-server#13