[Annotations] Encapsulate text tool styles using sass nesting

[jaredhirsch]

Right now, there are a lot of styles in static/css/frame.scss that are specific to the text editor tool.

Instead of those styles being declared at top level, it would be nice to use the fact that each tool puts a different class name on the #canvas-container element, and use sass nesting to encapsulate tool-specific styles.

For instance, the color styles currently scoped to #text-input, like:

#text-input.white {
  @include type-color($bg-black, white);
}

seem to work fine if nested under the more general .textTool selector:

.textTool {
  .white {
    @include type-color($bg-black, white);
  }
}

Related to #4700.