Slightly clean up registration of GPU convolution optimizers

As discussed [on theano-dev](https://groups.google.com/forum/#!topic/theano-dev/QEB-KAaMT-Q), this changes the registration of GPU-based convolution optimizers in the following way:
* The optimizer moving convolutions to GPU is pulled out of the `LocalGroupDB` named `conv_groupopt` in sandbox/cuda/opt.py
* `conv_groupopt` now only includes the optimizers replacing the legacy convolution with alternative implementations; they are registered via a group so they are tried in a specific order and we can control which ones to prefer.
* The tags they are registered with do not include the `local_` prefix any more. They are user-facing, so they should be user-friendly. The two FFT-related optimizers are registered under a common tag `conv_fft` to make it easier to include or exclude them.
* The `local_conv_dnn` optimizer of sandbox/cuda/dnn.py is only registered as part of `conv_groupopt`, not on its own any more. Otherwise we lose control over the order the optimizers are tried in.

If you're fine with the tags, let me know and I'll update the documentation accordingly in a separate commit.