Backend email filtering

[dmos62]

Fixes #410

Implements EmailDomainEquals and EmailDomainContains string matching predicates that are applyable only to email Mathesar types.

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[kgodey]

@dmos62 Please make the base branch of this PR whatever you branched off of instead of master.

[kgodey]

@dmos62 Can we close this PR since it's pretty out of date?