Support case insensitive 'term' for glossary references

[ideasman42]

Previously the glossary in sphinx was case insensitive, so foobar would properly link to Foobar in the glossary.
In sphinx-build 3.0.3 we're getting many warnings:

manual/addons/materials/material_library.rst:41: WARNING: term fake user not found in case sensitive match.made a reference to Fake User instead.
manual/animation/actions.rst:84: WARNING: term walk cycle not found in case sensitive match.made a reference to Walk Cycle instead.
manual/animation/armatures/bones/editing/bone_roll.rst:10: WARNING: term Euler rotation not found in case sensitive match.made a reference to Euler Rotation instead.
manual/animation/armatures/posing/bone_constraints/inverse_kinematics/spline_ik.rst:53: WARNING: term roll not found in case sensitive match.made a reference to Roll instead.
manual/animation/constraints/interface/common.rst:20: WARNING: term object origin not found in case sensitive match.made a reference to Object Origin instead.
manual/animation/constraints/interface/common.rst:22: WARNING: term mesh not found in case sensitive match.made a reference to Mesh instead.
manual/animation/constraints/interface/common.rst:22: WARNING: term lattice not found in case sensitive match.made a reference to Lattice instead.
manual/animation/constraints/interface/common.rst:34: WARNING: term head not found in case sensitive match.made a reference to Head instead.
manual/animation/constraints/interface/common.rst:34: WARNING: term tail not found in case sensitive match.made a reference to Tail instead.
manual/compositing/types/color/z_combine.rst:39: WARNING: term Anti-Aliasing not found in case sensitive match.made a reference to Anti-aliasing instead.
... snip ... (over 100 of these)

Describe the solution you'd like
An option to use case insensitive references without warning.

Describe alternatives you've considered
Adding different cases to the glossary, however this adds quite a bit
of redundancy with no real benefit.

Additional context

The current glossary we're maintaining https://docs.blender.org/manual/en/dev/glossary/index.html

[tk0miya]

Please set suppress_warnings = ['ref.term']. Then warnings will be suppressed.
Thanks,

[ideasman42]

This suppresses all ref.term warnings including invalid references that don't exist, which I don't want to suppress.

[polyvertex]

@tk0miya would it be reasonable to have at least an option added to the glossary directive?

Something like the following:

.. glossary::
    :caseinsensitive:

    [...]

[ideasman42]

@polyvertex that would be fine.

[tk0miya]

@polyvertex I need to know how to implement the option. I can accept your idea if a good PR is submitted.