Add support for Plex two-factor authentication

[chazlarson]

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Saltbox should handle Plex TFA cleanly.

Implementation ideas are discussed here:
https://forums.plex.tv/t/authenticating-with-plex/609370

This comes into play when running the initial tag when the server is claimed and in the plex_token.sh script. Seems like this interactive bit could be added up front in the install, though perhaps we can't tell if we'll actually need it until later.

In any case, wanted to get this into the backlog so it isn't forgotten. Personally, I'd like to turn on TFA and leave it on.

[saltydk]

Added in 09076cd