Rename flask-restplus -> flask-restx

[SteadBytes]

All occurrences of Flask-RESTPlus, flask-restplus e.t.c. should be replaced with Flaks-RESTX, flask-restx e.t.c. In addition, the Flask-RESTPlus logo should also be removed until a Flask-RESTX logo is created.

[SteadBytes]

I think this can be closed now, unless anyone can spot any that I've missed! 😄 @ziirish , @j5awry I'd appreciate if you could give the codebase a once-over (grep counts 😉 ) to see if there's anything that slipped through 😄

[ziirish]

Seems good to me now.
I spotted the configuration variables while working on the logo, I thought this was expected at first.

[SteadBytes]

No worries @ziirish ! Thanks for taking a look 😄