Do not allow words that are digits

[nya-elimu]

Make it impossible to create/edit words that are digits.

To prevent numbers from mistakenly being added as words (e.g. "10" instead of 10), display an error message if he content creator tries to add digits as a word.

In this example, "ten" should be added as a Word, and 10 should be added as a Number.

Numbers should be added to /content/number/list instead.

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Relates to #420

[nya-elimu]

@watr-sheep Would you be interested in having a look at this issue? 🙂

It relates to #420, which you implemented earlier.

[tomaszsmy]

#1583