rename `fe` and `be` in pagination_mode?

[chriddyp]

• fe and be are too terse
• Many users aren't necessarily aware of the development terms "front-end" or "back-end"
• However, the name will need to refer to these concepts in some way
• Alternatives:
  • client / server
  • browser / callback
  • frontend / backend

[alexcjohnson]

native / custom FTW #446