closes#8143
Fixed a potential issue (edge-case), where our generated and validated (in terms of check for existance and add a counter) would return a slug, that will exceed the maximum length of the slug fields (191 chars).
This is mostly possible for the post title, which can be 255 chars long and would generate a slug with the same length. This would prevent the user from actually saving a post.
I tried first to determine the expected length for a slug that already exists, but decided that the **easier** and simplyfied implementation is to always cut a slug to **185 chars** (+ counter). This makes it easier to find duplicates and includes a possible high number of counts (edge-edge-case).
The slug will not be cut down to 185 chars if it's an import.
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