Make user search case insensitive

[ianroberts]

Following on from #406, user search in the admin view should also be case-insensitive.

[github-actions]

Jest Coverage

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	83.8	83.96	64	83.8

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	83.8	83.96	64	83.8
_jrpc	94.11	91.66	83.33	94.11
_ index.js	94.11	91.66	83.33	94.11	29-30,38-40
_utils	81.97	82.97	57.89	81.97
_ annotations.js	97.72	73.91	100	97.72	35-36
_ dict.js	88.88	83.33	100	88.88	3-4
_ expressions.js	80.08	82.35	80	80.08	...,188-190,201-218
_ index.js	73.6	100	14.28	73.6	...4-65,76-82,93-94