Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/themes/classy/templates/user/forum-submitted.html.twig

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+{#
+/**
+ * @file
+ * Theme override for a forum post submission string.
+ *
+ * The submission string indicates when and by whom a topic was submitted.
+ *
+ * Available variables:
+ * - author: The author of the post.
+ * - time: How long ago the post was created.
+ * - topic: An object with the raw data of the post. Potentially unsafe. Be
+ *   sure to clean this data before printing.
+ *
+ * @see template_preprocess_forum_submitted()
+ */
+#}
+{% if time %}
+  <span class="submitted">{% trans %}By {{ author }} {{ time }} ago{% endtrans %}</span>
+{% else %}
+  {{ 'n/a'|t }}
+{% endif %}

core/themes/classy/templates/user/user.html.twig

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+{#
+/**
+ * @file
+ * Theme override to present all user data.
+ *
+ * This template is used when viewing a registered user's page,
+ * e.g., example.com/user/123. 123 being the user's ID.
+ *
+ * Available variables:
+ * - content: A list of content items. Use 'content' to print all content, or
+ *   print a subset such as 'content.field_example'.
+ * - Field variables: For each field attached to the user a corresponding
+ *   variable is defined; e.g., account.field_example has a variable
+ *   'field_example' defined. When needing to access a field's raw values,
+ *   developers/themers are strongly encouraged to use these variables.
+ *   Otherwise they will have to explicitly specify the desired field language,
+ *   e.g. account.field_example.en, thus overriding any language negotiation
+ *   rule that was previously applied.
+ * - attributes: HTML attributes for the container element.
+ * - user: A Drupal User entity.
+ *
+ * @see template_preprocess_user()
+ */
+#}
+<article{{ attributes.addClass('profile') }}>
+  {% if content %}
+    {{- content -}}
+  {% endif %}
+</article>

core/themes/classy/templates/user/username.html.twig

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+{#
+/**
+ * @file
+ * Theme override for displaying a username.
+ *
+ * Available variables:
+ * - account: The full account information for the user.
+ * - name: The user's name, sanitized.
+ * - extra: Additional text to append to the user's name, sanitized.
+ * - link_path: The path or URL of the user's profile page, home page,
+ *   or other desired page to link to for more information about the user.
+ * - link_options: Options to pass to the url() function's $options parameter if
+ *   linking the user's name to the user's page.
+ * - attributes: HTML attributes for the containing element.
+ *
+ * @see template_preprocess_username()
+ */
+#}
+{% if link_path -%}
+  <a{{ attributes.addClass('username') }}>{{ name }}{{ extra }}</a>
+{%- else -%}
+  <span{{ attributes }}>{{ name }}{{ extra }}</span>
+{%- endif -%}