Support for metrics and custom fields in expressions

frontend/src/metabase/lib/expressions/formatter.js

frontend/src/metabase/lib/expressions/index.js

frontend/src/metabase/lib/expressions/parser.js

.../metabase/query_builder/components/AggregationPopover.jsx

...c/metabase/query_builder/components/AggregationWidget.jsx

frontend/src/metabase/query_builder/components/FieldName.jsx

...lder/components/expressions/ExpressionEditorTextfield.jsx

...base/query_builder/components/expressions/Expressions.jsx

frontend/test/unit/lib/expressions.spec.js

frontend/test/unit/lib/expressions/formatter.spec.js

frontend/test/unit/lib/expressions/parser.spec.js