erp5_*: Replace data url by proper images on login_form

   The portal_skins was choosen to store the images to use proper http
cache.

bt5/erp5_oauth_facebook_login/SkinTemplateItem/portal_skins/erp5_oauth_facebook_login/facebook_login_button.png.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Image" module="OFS.Image"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_Cacheable__manager_id</string> </key>
+            <value> <string>anonymous_http_cache</string> </value>
+        </item>
+        <item>
+            <key> <string>__name__</string> </key>
+            <value> <string>facebook_login_button.png</string> </value>
+        </item>
+        <item>
+            <key> <string>content_type</string> </key>
+            <value> <string>image/png</string> </value>
+        </item>
+        <item>
+            <key> <string>height</string> </key>
+            <value> <int>36</int> </value>
+        </item>
+        <item>
+            <key> <string>precondition</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>width</string> </key>
+            <value> <int>191</int> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>

bt5/erp5_oauth_google_login/SkinTemplateItem/portal_skins/erp5_oauth_google_login/google_login_button.png.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Image" module="OFS.Image"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_Cacheable__manager_id</string> </key>
+            <value> <string>anonymous_http_cache</string> </value>
+        </item>
+        <item>
+            <key> <string>__name__</string> </key>
+            <value> <string>google_login_button.png</string> </value>
+        </item>
+        <item>
+            <key> <string>content_type</string> </key>
+            <value> <string>image/png</string> </value>
+        </item>
+        <item>
+            <key> <string>height</string> </key>
+            <value> <int>138</int> </value>
+        </item>
+        <item>
+            <key> <string>precondition</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>width</string> </key>
+            <value> <int>191</int> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>

bt5/erp5_openid_connect_client_login/SkinTemplateItem/portal_skins/erp5_openid_connect_client/openid_login_button.png.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Image" module="OFS.Image"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_Cacheable__manager_id</string> </key>
+            <value> <string>anonymous_http_cache</string> </value>
+        </item>
+        <item>
+            <key> <string>__name__</string> </key>
+            <value> <string>openid_login_button.png</string> </value>
+        </item>
+        <item>
+            <key> <string>content_type</string> </key>
+            <value> <string>image/png</string> </value>
+        </item>
+        <item>
+            <key> <string>height</string> </key>
+            <value> <int>37</int> </value>
+        </item>
+        <item>
+            <key> <string>precondition</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>width</string> </key>
+            <value> <int>191</int> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>

bt5/erp5_web_renderjs_ui/SkinTemplateItem/portal_skins/erp5_web_renderjs_ui/erp5_small_logo.png.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Image" module="OFS.Image"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_Cacheable__manager_id</string> </key>
+            <value> <string>anonymous_http_cache</string> </value>
+        </item>
+        <item>
+            <key> <string>__name__</string> </key>
+            <value> <string>erp5_small_logo.png</string> </value>
+        </item>
+        <item>
+            <key> <string>content_type</string> </key>
+            <value> <string>image/png</string> </value>
+        </item>
+        <item>
+            <key> <string>height</string> </key>
+            <value> <int>53</int> </value>
+        </item>
+        <item>
+            <key> <string>precondition</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>width</string> </key>
+            <value> <int>127</int> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>