Enhance BaseToggle with additional enabled/disabled toggle methods

Enhance BaseToggle with additional methods.

Add the following:

• is_disabled: Returns not is_enabled.
• is_toggled_on: Returns is_enabled.
• is_toggled_off: Returns not is_enabled.

This should make a variety of conditions more readable when ENABLE_ or DISABLE_ are in the toggle name.

Example 1: is_disabled

# BEFORE: slightly less readable
not SOME_FEATURE_TOGGLE.is_enabled()

# AFTER: more clear
SOME_FEATURE_TOGGLE.is_disabled()  

Example 2: is_toggled_off

# BEFORE: brain twister
not DISABLE_SOME_FEATURE.is_enabled()

# AFTER: more clear
DISABLE_SOME_FEATURE.is_toggled_off()  

Notes:

• For a DISABLE_FEATURE toggle, the toggle is off when the feature is on, so we use _toggle_ in the method name is_toggled_off to clearly refer to the toggle and not the feature.
• For docs, possibly add an “Other ways to check a toggle” (or something like that) after https://github.com/openedx/edx-toggles/blob/master/docs/how_to/implement_the_right_toggle_type.rst#accessing-toggles?
• Similar change is needed for CourseWaffleFlag in edx-platform, which is slightly different because it takes an argument for is_enabled.

Questions:

• Is this in fact simpler to understand? Are there better options?
  • We've decided to move forward

Work required:

• https://github.com/openedx/edx-toggles/pull/293
• Update docs (see notes)
• Update CourseWaffleFlag in edx-platform (see notes).

Start in edx_toggles/toggles/internal/base.py and review the linked access-toggles documentation section. Check pull request 293 and the CourseWaffleFlag reference in edx-platform before starting. Done means the requested BaseToggle methods, documentation updates, and corresponding CourseWaffleFlag work are addressed.