Make sure translators enter valid slug of the first post

[nbachiyski]

Currently the slug of the first post is taken by the call:

_x('hello-world', 'Default post slug')

Then, this value is directly inserted in the database. If the translator entered a string in a non-sanitized form, the first post just won't open with pretty permalinks on.

If we call sanitize_title() in the string, we can make sure the right value goes in the database.

[ryan]

(In [12784]) Sanitize translation of default post slug. Props nbachiyski. fixes #11952