minor #3170 [3.0] SlugField: don't translate characters & remove special characters (jmsche)
minor #3170 [3.0] SlugField: don't translate characters & remove special characters (jmsche)

This PR was squashed before being merged into the 3.0.x-dev branch.

Discussion
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[3.0] SlugField: don't translate characters & remove special characters

Follow-up of Javier's comment: https://github.com/EasyCorp/EasyAdminBundle/pull/3164#issuecomment-622336387

This PR:

* extends (only once) the slugify() function to remove translated symbols (eg. "$" => "dollar")
* uses the "lower" option of slugify instead of .toLowerCase()
* allows only alphanumeric characters (and dashes) in the slug

Could supersede https://github.com/EasyCorp/EasyAdminBundle/commit/ec3c8da9508b063f719783eb87d52faee977487f ?

Commits
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8f8e10c5 [3.0] SlugField: don't translate characters & remove special characters

javiereguiluz/EasyAdminBundle,javiereguiluz/EasyAdminBundle