Crop string is length is more than a maximum

In the Child Safety project we need to ensure that the name of the child is no longer than a specified value. As we need to apply this constrain everywhere in the whole project I thought that it might be a good candidate for including it in the StringUtils module in the NoTils project, instead of creating our own static helper method.

- Add a new function that crops a string if its length is more than a maximum value and appends "..." at its end.
- Tests implemented