Do stricter check using regex to validate hex UUID value

ValueError: invalid literal for int() with base 16: '//..//..//..//../WEBINF/web.xml\x00'

Sentry Issue: KOLIBRI-BACKEND-2D5

ValueError: invalid literal for int() with base 16: '//..//..//..//../WEBINF/web.xml\x00'
(19 additional frame(s) were not displayed)
...
  File "django/db/models/lookups.py", line 170, in as_sql
    rhs_sql, rhs_params = self.process_rhs(compiler, connection)
  File "django/db/models/lookups.py", line 103, in process_rhs
    return self.get_db_prep_lookup(value, connection)
  File "django/db/models/lookups.py", line 196, in get_db_prep_lookup
    [get_db_prep_value(value, connection, prepared=True)]
  File "morango/models/fields/uuids.py", line 39, in get_db_prep_value
    value = uuid.UUID(value)
  File "uuid.py", line 178, in __init__
    int = int_(hex, 16)

Start in morango/models/fields/uuids.py at get_db_prep_value, where the traceback shows the value is converted with uuid.UUID. Review the existing conversion and add the requested regex-based hexadecimal UUID validation so malformed values such as the reported path are rejected before conversion; done means the invalid input no longer reaches the failing conversion.