Add proxy settings

[LivInTheLookingGlass]

It would be nice if I could use this app proxied through Orbot. I cannot do this unless it supports either HTTP or SOCKS proxies. I tried using it through Orbot's VPN mode, and it worked, but I cant use the VPN all the time since I also need it for work.

[werman]

I've started to work on it.
Work scope:

• Add option with proxy address/port, selection between HTTP and SOCKS, login/password, connection testing
• Make all requests through okhttp library to have central proxy settings

[werman]

Added proxies support in #356

There are some cosmetic things left like showing some progress when testing proxy and displaying that proxy enabled in settings without opening the dialog.

[LivInTheLookingGlass]

Awesome! Cant wait for this to be merged