[Feature] Add Proxy Support

[ghbjklhv]

Feature: Add proxy support to settings.
Why?: Many reasons, in my case this would be useful for routing Mastadon through TOR but it could also be used for I2p or a regular clear net proxy or anything else that supports proxy.
It would also allow users to connect to a .onion and I2p Mastadon instances.

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[alicerunsonfedora]

We'll definitely take a look, though proxy support probably won't make it to the web version. It may be possible with the desktop apps via some Electron APIs, though I can't guarantee it.