Allow customizing fonts and sizes

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Some of the strings that Jami displays are in fonts that are too small.
It makes them hard to read.  Could you please get them all made larger?

I agree that some of the fonts are too small, and I've received complaints about them from a few folks with various degrees of visual impairment.

While true that it may be possible to change the fonts/sizes using system-wide settings and/or using the QT_SCALE_FACTOR, it would be nice to allow the user to override system defaults and customize them for Jami inside the application as well for more convenience.