💡 Allow setting custom fonts

[thermosflasche]

⚬ PROBLEM: The font changer requested on #209 (located at General > More > Font on the UI) lets you choose a font among a fixed list (Comfortaa, Lato, Merriweather...).

⚬ SOLUTION: Allow the user to specify a custom font by typing its name.

First solution that came to mind: add a "Other..." option on the checklist, which, if selected, shows an input box below the "Font" option, maybe with a placeholder like "Liberation Sans"

⚬ ALTERNATIVES:

• Overriding CSS using other extensions like Stylish

⚬ RELEVANCE / SCOPE: I think that, particularly, users interested in ricing their web experience would love this feature! :-)

Also, it would indirectly solve feature requests for font introducing, like #2793, #2912 and so on.

⚬ "SIDE EFFECTS": It would make the "Settings" and "Font" lists a little longer, but only if the user chooses to.

⚬ CONTEXT: Users should verify that the font is accessible from their environment, whether they defined a built-in font, or a font locally installed by them.
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Thank you!

SHORT Table	(Summary)
Problem	Limited font options in the "Font" list
Solution	Allow user to choose a custom font
Alternatives	Override CSS font with extensions like Stylish
Scope	Customization ("ricing"), less feature requests for adding fonts
Side effects	Larger "Font" list, and possibly an extra textbox in the Settings > General list
Context	Is the user able to access specified font?