Refactor several "field grids" per Buefy theme

e.g. the Users field when viewing a Role, and Vendor Sources panel
when viewing a Product

tailbone/grids/core.py

@@ -1418,12 +1418,13 @@ class GridAction(object):
     """
 
     def __init__(self, key, label=None, url='#', icon=None, target=None,
-                 click_handler=None):
+                 link_class=None, click_handler=None):
         self.key = key
         self.label = label or prettify(key)
         self.icon = icon
         self.url = url
         self.target = target
+        self.link_class = link_class
         self.click_handler = click_handler
 
     def get_url(self, row, i):