[2.x] Support Chinese slug

[thingasd]

Modify the slugify function to accept Chinese character as slug, it should work with setting appropriate config twill.slug_utf8_languages

[haringsrob]

Holding this one back for testing with https://github.com/area17/twill/pull/1897

[haringsrob]

Hey @thingasd

Could you tell me what is expect to be the slug for when I for example enter:

• test-知道
• 知道
• 知道bar知道

Kind regards,

[thingasd]

Hey @thingasd

Could you tell me what is expect to be the slug for when I for example enter:

• test-知道
• 知道
• 知道bar知道

Kind regards,

test-知道 知道 知道bar知道

[haringsrob]

ok, so slugs can actually contain chinese characters?

[thingasd]

ok, so slugs can actually contain chinese characters?

This pr allows slugs to contain Chinese characters, which can be used in the URL for improving the readability. i.e. http://www.example.com/例子