558 access token for private repositories

This shall fix #558. As soon as hets (https://github.com/spechub/Hets/issues/1422) can handle it completely, this should work. Now, it is working for non-owl files whose file type can be detected by hets' magic.

After merging, don't forget to put this line into the crontab for the expired tokens to be deleted daily:
```
0 0  * * *  ~/current/script/rails runner 'AccessTokenDeletionWorker.new.perform'
```
couldn't this be done by the `create :access_token` call? or do you utilize the lazy-evaluation here - if so, please add a comment?