add classnames template tag for generating classnames

- use classnames template tag in shared header template
- add classname as documented variable for the shared header template

CHANGELOG.txt

@@ -54,6 +54,7 @@ Changelog
  * Prefetch workflow states in edit page view to to avoid queries in other parts of the view/templates that need it (Tidiane Dia)
  * Remove the edit link from edit bird in previews to avoid confusion (Sævar Öfjörð Magnússon)
  * Introduce new template fragment and block level enclosure tags for easier template composition (Thibaud Colas)
+ * Add a `classnames` template tag to easily build up classes from variables provided to a template (Paarth Agarwal)
  * Fix: Typo in `ResumeWorkflowActionFormatter` message (Stefan Hammer)
  * Fix: Throw a meaningful error when saving an image to an unrecognised image format (Christian Franke)
  * Fix: Remove extra padding for headers with breadcrumbs on mobile viewport (Steven Steinwand)

docs/releases/4.0.md

@@ -61,6 +61,7 @@ When using a queryset to render a list of images, you can now use the `prefetch_
  * Prefetch workflow states in edit page view to to avoid queries in other parts of the view/templates that need it (Tidiane Dia)
  * Remove the edit link from edit bird in previews to avoid confusion (Sævar Öfjörð Magnússon)
  * Introduce new template fragment and block level enclosure tags for easier template composition (Thibaud Colas)
+ * Add a `classnames` template tag to easily build up classes from variables provided to a template (Paarth Agarwal)
 
 ### Bug fixes
 

wagtail/admin/templates/wagtailadmin/shared/header.html

@@ -3,6 +3,7 @@
 
     Variables accepted by this template:
 
+    - `classname` - if present, adds classname to the header class list. This will be the class/classes added to the header class list
     - `title` - Displayed as `h1`
     - `subtitle` - Within the `h1` tag but smaller
     - `search_url` - if present, display a search box. This is a URL route name (taking no parameters) to be used as the action for that search box
@@ -14,7 +15,7 @@
     - `action_icon` - icon for the 'action' button, default is 'icon-plus'
 
 {% endcomment %}
-<header class="header {% if merged %}merged{% endif %} {% if search_form %}hasform{% endif %}">
+<header class="{% classnames "header" classname merged|yesno:"merged," search_form|yesno:"hasform," %}">
     {% block breadcrumb %}{% endblock %}
     <div class="row">
         <div class="left">

wagtail/admin/templatetags/wagtailadmin_tags.py

@@ -153,6 +153,14 @@ def page_permissions(context, page):
     return _get_user_page_permissions(context).for_page(page)
 
 
+@register.simple_tag
+def classnames(*classes):
+    """
+    Returns any args as a space-separated joined string for using in HTML class names.
+    """
+    return " ".join([classname.strip() for classname in classes if classname])
+
+
 @register.simple_tag(takes_context=True)
 def test_collection_is_public(context, collection):
     """