helpers: Consolidate MakeSegment vs MakePathSanitized

In short:

* Avoid double tolower in MakeSegment
* Use MakePathSanitized for taxonomies in pageToPermalinkTitle; this matches what MakeSegment does.
* Move the "double hyphen and space" logic into UnicodeSanitize

The last bullet may be slightly breaking for some that now does not get the "--" in some URLs, but we need to reduce the amount of URL logic.

See #4926

helpers/path_test.go

@@ -51,7 +51,8 @@ func TestMakeSegment(t *testing.T) {
 		{"Your #1 Fan", "your-1-fan"},
 		{"Red & Blue", "red-blue"},
 		{"double//slash", "double-slash"},
-		{"My // Taxonomy", "my-taxonomy"},
+		{"triple///slash", "triple-slash"},
+		{"-my/way-", "my-way"},
 	}
 
 	for _, test := range tests {