Use getDisplayName() for user names consistently

Problem/Motivation

• Realname module alters the display name, but at user/1234 the username is shown. This is incorrect.
• Tons of places in core that show the username, but need to show the users display name.
• Comments do not show DisplayName for registered users.
• Security problem as usernames should never shown on websites. With real names you cannot login and try passwords, with a username you can.

Proposed resolution

• Fix all places where getUsername() or getAccountName() is used, but getDisplayName() should be used.
• Change core to enable a DisplayName by default. That is why tons of tests are failing because of wrong usage.

Remaining tasks

Allow for configurable truncation length of display name: #2767787: Allow custom truncate username settings

User interface changes

Users DisplayName will be properly shown.

API changes

API addition: Entity::getDisplayNameTruncated to get the correct abrivation used in theme_preprocess_username() function.

Data model changes

none