Add prepare_url_slash to string_helpers.py and tests

Function cleans up url paths (without domain) by ensuring they start with a single slash and removing double slashes.

src/corelibs/string_handling/string_helpers.py

@@ -2,6 +2,7 @@
 String helpers
 """
 
+import re
 from decimal import Decimal, getcontext
 from textwrap import shorten
 
@@ -101,4 +102,21 @@ def format_number(number: float, precision: int = 0) -> str:
         "f}"
     ).format(_number)
 
+
+def prepare_url_slash(url: str) -> str:
+    """
+    if the URL does not start with /, add slash
+    strip all double slashes in URL
+
+    Arguments:
+        url {str} -- _description_
+
+    Returns:
+        str -- _description_
+    """
+    url = re.sub(r'\/+', '/', url)
+    if not url.startswith("/"):
+        url = "/" + url
+    return url
+
 # __END__