add `pref_outgoing_media_quality_hint` below "Outgoing Media Quality" selection

[r10s]

once the strings are pulled and pref_outgoing_media_quality_hint is available, add the string between the "Outgoing Media Quality" options and buttons.

the style should be same as used for "Disappearing Messages" (which has a similar hint), so just plain, normal, boring text.

cmp deltachat/deltachat-android#4259 and deltachat/deltachat-ios#3046 (mobile has a different font style for these hints, however, on desktop, for now stay with what is used elsewhere - there is space and it looks good at "Disappearing Messages")