jsxc: Update content security policy to prevent style errors

- Without the CSP, during loading there are no errors in the console. However,
during chatting, some styling related error show up.

Tests:

- Ensure that there are no CSP related errors in the browser console.

Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/jsxc/views.py

@@ -1,10 +1,13 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 """Views for the JSXC module."""
 
+import copy
+
 from django.http import Http404
 from django.views.generic import TemplateView
 
 import plinth.app as app_module
+from plinth.middleware import CONTENT_SECURITY_POLICY
 from plinth.modules.names.components import DomainName
 
 
@@ -12,6 +15,14 @@ class JsxcView(TemplateView):
     """A simple page to embed Javascript XMPP Client library."""
 
     template_name = 'jsxc_launch.html'
+    headers: dict[str, str] = {}
+
+    def __init__(self, **kwargs):
+        """Initialize the view and set CSP."""
+        super().__init__(**kwargs)
+        csp = copy.copy(CONTENT_SECURITY_POLICY)
+        csp['style-src'] = "'self' 'unsafe-inline'"
+        self.headers['Content-Security-Policy'] = csp.get_header_value()
 
     def dispatch(self, request, *args, **kwargs):
         """Don't serve the view when app is disabled."""
@@ -26,3 +37,8 @@ class JsxcView(TemplateView):
         context = super().get_context_data(*args, **kwargs)
         context['domain_name'] = DomainName.list_names()[0]
         return context
+
+    def get(self, request, *args, **kwargs):
+        """Handle GET request and return a response object."""
+        context = self.get_context_data(**kwargs)
+        return self.render_to_response(context, headers=self.headers)