Extract the scalars dashboard to a proper plugin

This CL achieves the following:
  - The output of the /data/runs endpoint no longer contains any
    references to "scalars."
  - The router no longer has any special handling for scalars.
  - The application (application.py) and its tests no longer have any
    special handling for scalars.

As part of the process of pulling out the scalars dashboard, this leaves
behind some assorted infrastructure to make it easier to migrate other
plugins (primarily, in the router).

Tests done so far:
  - All tests descended from the "tensorboard" directory
  - Manual interaction of the following forms: expanding a scalar plot;
    enlarging a scalar plot; shrinking a scalar plot; using the run
    selector's check boxes and radio buttons; using the run selector's
    regex filter; creating a tag group; viewing and scrolling through
    images, along with a casual check of the other dashboards
  - Manual inspection of the /data/runs output to make sure that it
    contains no scalar data

PiperOrigin-RevId: 157028690