Remove user OAuth tokens stored in database for Bitbucket, GitHub, and GitLab

and request them each session. Pass these tokens to the project import data.

This prevents the need to encrypt these tokens and clear them in case they
expire or get revoked.

For example, if you deleted and re-created OAuth2 keys for Bitbucket, you would get
an Error 500 with no way to recover:

```
Started GET "/import/bitbucket/status" for x.x.x.x at 2015-08-07 05:24:10 +0000
Processing by Import::BitbucketController#status as HTML
Completed 500 Internal Server Error in 607ms (ActiveRecord: 2.3ms)

NameError (uninitialized constant Import::BitbucketController::Unauthorized):
  app/controllers/import/bitbucket_controller.rb:77:in `rescue in go_to_bitbucket_for_permissions'
  app/controllers/import/bitbucket_controller.rb:74:in `go_to_bitbucket_for_permissions'
  app/controllers/import/bitbucket_controller.rb:86:in `bitbucket_unauthorized'
```

Closes #1871

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