Remove polynomial regexps

https://github.com/dimagi/commcare-hq/security/code-scanning/273
https://github.com/dimagi/commcare-hq/security/code-scanning/361
https://github.com/dimagi/commcare-hq/security/code-scanning/363

 isdigit() method of builtins.str instance
     Return True if the string is a digit string, False otherwise.

     A string is a digit string if all characters in the string are digits and there
     is at least one character in the string.

The commtrack one is well covered by tests, the others I verified
locally

corehq/apps/commtrack/util.py

@@ -193,8 +193,12 @@ def encode_if_needed(val):
 
 
 def _fetch_ending_numbers(s):
-    matcher = re.compile(r"\d*$")
-    return matcher.search(s).group()
+    postfix = ''
+    for char in s[::-1]:
+        if not char.isdigit():
+            break
+        postfix = char + postfix
+    return postfix
 
 
 def generate_code(object_name, existing_codes):

corehq/apps/registration/forms.py

@@ -232,7 +232,7 @@ def clean_phone_number(self):
         phone_number = re.sub(r'\s|\+|\-', '', phone_number)
         if phone_number == '':
             return None
-        elif not re.match(r'\d+$', phone_number):
+        elif not phone_number.isdigit():
             raise forms.ValidationError(gettext(
                 "%s is an invalid phone number." % phone_number
             ))

corehq/apps/users/views/mobile/users.py

@@ -385,7 +385,7 @@ def post(self, request, *args, **kwargs):
         if self.request.POST['form_type'] == "add-phonenumber":
             phone_number = self.request.POST['phone_number']
             phone_number = re.sub(r'\s', '', phone_number)
-            if re.match(r'\d+$', phone_number):
+            if phone_number.isdigit():
                 is_new_phone_number = phone_number not in self.editable_user.phone_numbers
                 self.editable_user.add_phone_number(phone_number)
                 self.editable_user.save(spawn_task=True)