front-end fixes for unlimited use cases

[hasan-ozbey]

Fixes #28

Everything should be fine in back-end, judging by this line here:

https://github.com/FriendsOfFlarum/doorman/blob/ab049df0c3c40d3612799d99990622bc78b8e948/src/Listeners/AddValidatorRule.php#L43

Changes proposed in this pull request: Added new locale key for suitability, that will fix possible "-1 Use left" alike issues.

Tested this commit locally.

[dsevillamartin]

Implemented in e43181d. Thanks!