text: Title: [BUG] 500 when invalid overwrite url is filled in
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  ## Description
  
  From time to time some of our clients manage to put random (invalid) strings as slug in the overwrite url (in advanced settings) which results in 500s. 
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  ## Steps to reproduce
  
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  1. Go to Advanced settings
  2. Enter an overwrite url containing an invalid character (for instance a space)
  
  ## Expected behaviour
  
  A validation error is raised that says it is an invalid url.
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  ## Actual behaviour
  
  The cms starts raising errors.:
  
  ```
  NoReverseMatch: Reverse for 'pages-details-by-slug' with keyword arguments '{'slug': 'this is an invalid slug'}' not found. 1 pattern(s) tried: ['(?P<slug>[0-9A-Za-z-_.//]+)/$']
  ```
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  ## Screenshots
  
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  ## Additional information (CMS/Python/Django versions)
  
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  ## Do you want to help fix this issue?
  
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  * [x] Yes, I want to help fix this issue and I will join #workgroup-pr-review on [Slack](https://www.django-cms.org/slack) to confirm with the community that a PR is welcome. 
  * [ ] No, I only want to report the issue.
  