Add erpnext-grey.png, add logo in footer (#10101)

* Add erpnext-grey.png, footer html

* Sent via ERPNext
diff --git a/erpnext/hooks.py b/erpnext/hooks.py
index 3a8569b..fc8d39b 100644
--- a/erpnext/hooks.py
+++ b/erpnext/hooks.py
@@ -201,11 +201,14 @@
 
 email_brand_image = "assets/erpnext/images/erpnext-logo.jpg"
 
-default_mail_footer = """<div style="text-align: center;">
-	<a href="https://erpnext.com?source=via_email_footer" target="_blank" style="color: #8d99a6;">
-		Sent via ERPNext
-	</a>
-</div>"""
+default_mail_footer = """
+	<span>
+		Sent via
+		<a class="text-muted" href="https://erpnext.com?source=via_email_footer" target="_blank">
+			ERPNext
+		</a>
+	</span>
+"""
 
 get_translated_dict = {
 	("doctype", "Global Defaults"): "frappe.geo.country_info.get_translated_dict"
diff --git a/erpnext/public/images/erpnext-grey.png b/erpnext/public/images/erpnext-grey.png
new file mode 100644
index 0000000..0e70f05
--- /dev/null
+++ b/erpnext/public/images/erpnext-grey.png
Binary files differ