Use mbstring-extensions as applicable

[joho1968]

I've just been browsing the code a bit to see if I can contribute, but one thing that I noticed is the use of some non-mbstring string functions.

Since Nextcloud requires the mbstring functions to be present in PHP, I think it'd be a good idea if the welcome app made use of them where applicable.

https://docs.nextcloud.com/server/latest/admin_manual/installation/source_installation.html

(see "Prerequisites for manual installation")

Just my two cents.

[kyteinsky]

The code with non-mbstring functions has been overwritten and since the new functions only handle URIs, I doubt there is any need for mbstrings here. (b9683f5#diff-04ace24b494c1cf87cb15970508f767e982403f9001ea2f5faa80e9f4247f55fR140)