make namespace filter multi select and change onChange

Signed-off-by: Sebastian Malton <sebastian@malton.name>

src/renderer/components/+namespaces/namespace-select.tsx

@@ -91,7 +91,8 @@ export class NamespaceSelectFilter extends React.Component {
         closeMenuOnSelect={false}
         isOptionSelected={() => false}
         controlShouldRenderValue={false}
-        onChange={({ value: namespace }: SelectOption) => toggleContext(namespace)}
+        isMulti
+        onChange={([{ value }]: SelectOption[]) => toggleContext(value)}
         formatOptionLabel={({ value: namespace }: SelectOption) => {
           const isSelected = hasContext(namespace);
           return (