Embed player element visibility control

[rpeterman-gp]

Allow elements to be hidden, such as thumbnail, title, subtitle, branding, menus and their individual buttons, and background image. This should allow creators to configure minimal versions of player to embed into their own pages as UI elements, without visually duplicating image and title of the episode's page.

This will need updates to the embed configuration interfaces, and model parser, along with layout styles and selection logic.

• Any element with an override parameter should support a 'hidden' value. Empty values should continue to use default values and fallbacks.
• Thumbnail: Hide when episode image is not available and no podcast image is available for fallback. Currently the image will not render, but the player layout leaves an empty space when missing.
• Title and Subtitle: Add layout to support not showing text. Subtitle should be hidden if title is hidden.
• Branding: add parameter hb to flag PRX logo to be hidden.
• Menu: add parameter hm. A value of 'hidden', 'all', or '1' should hide the entire menu. Value can be the name of an individual submenu or menu button. Multiple elements can be hidden by providing additional hm parameters.
• Background Image: This should already be not rendered when an image URL is not provided. May need to adjust color scheme background colors when image is missing.