Add support for User entities

Problem/Motivation

We want to support all core content entity types!

This issue is to add a 'PanelizerEntity' plugin to provide support for 'user' entities.

Here's a rough idea of the process:

• Create the plugin class in src/Plugin/PanelizerEntity/ directory
• See src/Plugin/PanelizerEntity/PanelizerNode.php for an example!
• If the entity has a 'view_builder', look at it's code for anything that makes the entity special
• If this entity needs any special blocks for it's default display, for example, like the 'node links' block, create issues in the CTools queue (so Page Manager could use them, if it wanted)
• Check for a hook_entity_extra_field_info() which could have some things that need special blocks
• Check for a hook_ENTITY_TYPE_view_alter() for other rendering wierdness we might need to replicate
• Check the support code in Panelizer for D7 (and CTools for D7, for special blocks) to see if we did anything special there that makes sense in D8 too
• Add tests

Proposed resolution

Add a 'PanelizerEntity' plugin to provide support for 'user' entities.

Remaining tasks

• Create the plugin class in src/Plugin/PanelizerEntity/ directory
• Extend plugin class with base functionality (see src/Plugin/PanelizerEntity/PanelizerNode.php for an example)
• If the entity has a 'view_builder', look at it's code for anything that makes the entity special see comment #3
• If this entity needs any special blocks for it's default display, for example, like the 'node links' block, create issues in the CTools queue (so Page Manager could use them, if it wanted)
• Check for a hook_entity_extra_field_info() which could have some things that need special blocks
• Check for a hook_ENTITY_TYPE_view_alter() for other rendering wierdness we might need to replicate
• Check the support code in Panelizer for D7 (and CTools for D7, for special blocks) to see if we did anything special there that makes sense in D8 too

User interface changes

API changes

Data model changes