gravatar: use background-image style instead of img element for gravatar

When having several 1000 users, chrome will fail to load all the gravatar img
elements on the _admin/users page (ERR_INSUFFICIENT_RESOURCES).

If instead we use a background-image style, chrome will only load the images of
the elements that are actually displayed.

kallithea/lib/helpers.py

@@ -878,8 +878,9 b" def gravatar(email_address, cls='', size"
     if src:
         # here it makes sense to use style="width: ..." (instead of, say, a
         # stylesheet) because we using this to generate a high-res (retina) size
-        html = ('<img alt="" class="{cls}" style="width: {size}px; height: {size}px" src="{src}"/>'
-            .format(cls=cls, size=size, src=src))
+        html = ('<i class="icon-empty {cls}"'
+                ' style="font-size: {size}px;background-size: {size}px;background-image: url(\'{src}\')"'
+                '></i>').format(cls=cls, size=size, src=src)
 
     else:
         # if src is empty then there was no gravatar, so we use a font icon

kallithea/public/css/contextbar.css

@@ -11,6 +11,7 b" i[class^='icon-'] {"
     margin: -2px 0 -4px 0;
     /* background-color: red; /* for debugging */
 }
+/* Note: class 'icon-empty' can be used to get this styling without an actual glyph */
 
 /* css classes for diff file status ... it'd be nice if css had a way to
    inherit from another class but alas, we must make sure this content is the

kallithea/public/js/base.js

@@ -1083,7 +1083,9 b' var gravatar = function(gravatar_lnk, si'
     if (gravatar_lnk == 'default') {
         return '<i class="icon-user {1}" style="font-size: {0}px;"></i>'.format(size, cssclass);
     }
-    return '<img alt="" class="{2}" style="width: {0}px; height: {0}px" src="{1}"/>'.format(size, gravatar_lnk, cssclass);
+    return ('<i class="icon-empty {2}"' +
+            ' style="font-size: {0}px;background-image: url(\'{1}\'); background-size: {0}px"' +
+            '></i>').format(size, gravatar_lnk, cssclass);
 }
 
 var autocompleteGravatar = function(res, gravatar_lnk, size, group) {