make avatar lookup occur at image request (#10540)

speed up page generation by making avatar lookup occur at the browser
not at page generation

* Protect against evil email address ".."

* hash the complete email address

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-Authored-By: Lauris BH <lauris@nix.lv>

modules/base/tool_test.go

@@ -90,17 +90,6 @@ func TestSizedAvatarLink(t *testing.T) {
 	)
 }
 
-func TestAvatarLink(t *testing.T) {
-	disableGravatar()
-	assert.Equal(t, "/img/avatar_default.png", AvatarLink("gitea@example.com"))
-
-	enableGravatar(t)
-	assert.Equal(t,
-		"https://secure.gravatar.com/avatar/353cbad9b58e69c96154ad99f92bedc7?d=identicon",
-		AvatarLink("gitea@example.com"),
-	)
-}
-
 func TestFileSize(t *testing.T) {
 	var size int64 = 512
 	assert.Equal(t, "512 B", FileSize(size))