Fix FK5 -> Galactic conversion

As discussed on the list, our FK5 -> Galactic conversion is not consistent with our FK5 -> FK4 -> Galactic conversion, and is not consistent with other codes. If we set the following attributes on Galactic:

```
coord.Galactic._ngp_J2000 = coord.FK4NoETerms(ra=192.25*u.degree, dec=27.4*u.degree).transform_to(coord.FK5)
coord.Galactic._lon0_J2000 = coord.Angle(122.931918559, u.degree)
```

then everything works fine and we agree much better with other codes (and are self-consistent). I can't claim to understand all the details and don't have time to look into it more for now, so would be useful to have input from others more familiar with coordinate conversions?

cc @eteq