Release 2.3.0 (#4412)

- move user-preferences to edit submenu
- unify user preferences with other forms
- use light fieldset for all user-edit-forms
- support offcanvas elements
- allow to dynamically inject toolbar buttons (via plugins)
- use interface for formatting dates
- improve docker issue template
- bump version and composer packages

src/Event/ThemeEvent.php

@@ -19,6 +19,7 @@ class ThemeEvent extends Event
     public const HTML_HEAD = 'app.theme.html_head';
     public const CONTENT_BEFORE = 'app.theme.content_before';
     public const CONTENT_START = 'app.theme.content_start';
+    public const TOOLBAR = 'app.theme.toolbar';
     public const CONTENT_END = 'app.theme.content_end';
     public const CONTENT_AFTER = 'app.theme.content_after';