Move the Flask dependency and proxy host code

[mdazam1942]

Move flask dependency and the proxy host code out of stix-shifter.py to somewhere else. We will add instructions to the developer doc to manually install Flask before running the proxy host.

[Ryan-Gordon1]

Hi @mdazam1942
For this issue are you just looking to have the Flask related code/routes a separate module which stix-shifter uses and same for proxy settings ?

[yurii-klymenko]

yes. The idea is to make stix-shifter working without flask dependency installed until it is started in webserver-proxy mode.

[DerekRushton]

This is being done with #1730. Closing this item.