Check for required mbstring extension

[colemanw]

No description provided.

[colemanw]

Grepping around our codebase, we have a habit of always using mbstring functions conditionally. This breaks with that pattern, mostly because of #9268. What do we think?

[colemanw]

I'm having second thoughts about this because SOAP is rarely used. I'm now inclined to:

• Keep this PR but downgrade it from "error" to "warning"
• Wrap the news widget's use of mb_substr in a conditional

[nganivet]

Without understanding much of the context ... our database is UTF-8, anyone can type anything in all text fields (ie. I could well enter Russian or Chinese names on any contribution form), and PHP notably breaks if we do not use the mb_* functions with such input.

So I would +1 mandating the mbstring extension as this PR suggests, but also wrap the use in a conditional to be consistent with the rest of the code base. Upvoted Coleman's 2nd suggestion.

[seamuslee001]

I am a + on the need for mbstring, I think that it should be needed as it is useful and since php4

[seamuslee001]

I am satisfied with Coleman's 2nd option.