Enable no-else-return rule

[ottaviohartman]

For #5456. Add no-else-return ESLint rule.

Change all instances of the form:

if (a) {
  return x;
else {
  return y;
}

to:

if (a) {
  return x;
}
return y;

[mramato]

Submitted tweak to move new rule to the shared config, but otherwise this looks great. Thanks @omh1280 !