Add User view mode template suggestions

The approach looks sound and adding a custom user--full.html.twig template worked for me.

A couple of things in the test:

1. +++ b/core/modules/user/src/Tests/UserTemplateSuggestionsTest.php
   @@ -0,0 +1,30 @@
   +    $view_mode = 'node.my_custom_view_mode';
   

   Did you mean node, or user? Either way, I'm not sure EntityViewBuilder::view() expects an entity name prepended like this, it looks a bit strange but it does work in this example.

2. +++ b/core/modules/user/src/Tests/UserTemplateSuggestionsTest.php
   @@ -0,0 +1,30 @@
   +    $build = \Drupal::entityManager()->getViewBuilder('user')->view($user, $view_mode);
   

   \Drupal::entityTypeManager() could be used as a straight swap for \Drupal::entityManager() here, to avoid the deprecated function call.

Also worth looking at https://www.drupal.org/node/2354645 to see the template naming conventions for other entities. They do all seem to have slightly different suggestions, but is there anything else to consider, e.g. user ID/bundle name?

The documentation on that page would need updating too.

Patch in #8 looks good.

This is a nice enhancement and brings theming of user entities more closely in line with other entities such as node/taxonomy etc.

This probably needs some input from the theme system maintainers, so tagging accordingly.

Mimics node suggestions in node_theme_suggestions_node() and has a test as well.

+++ b/core/modules/user/user.module
@@ -1426,3 +1426,13 @@ function template_preprocess_user(&$variables) {
+  $suggestions = array();

Let's use array square bracket syntax.

Thanks @claudiu.cristea, I've just changed the arrays to short syntax and added the test method public visibility.

Moving back to CNR for the same reason as #2767243: Create a theme suggestion for taxonomy terms by view mode.

Sorry folks, but I think this should be fixed here instead: #2270883: Automatically add theme hook suggestions for all entity types