These links were valid in v1beta2 but when the upgrade to
v1 occurred they started to give 404s. Fixes googleapis#1211.

Replace mostly with

```
git grep -l v1beta2 -- gcloud/pubsub | xargs sed -i s/v1beta2/v1/g
git grep -l 'v1/projects/' -- gcloud/pubsub | \
    xargs sed -i 's/v1\/projects\//v1\/projects\./g'
```

but manually replaced

```
/pubsub/reference/rest/v1beta2/projects/topics/subscriptions/list
   -> /pubsub/reference/rest/v1/projects.topics.subscriptions/list

/pubsub/reference/rest/google/pubsub/v1beta2/PubsubMessage
   -> /pubsub/reference/rest/v1/PubsubMessage
```

after the fact.