github / gitlab subscribers

- added commands for subscribing / unsubscribing from pull requests / merge requests for a given user:

```
bender github s(ubscribe) some-project some-user
bender gitlab s(ubscribe) some-project some-user
bender github uns(ubscribe) some-project some-user
bender gitlab uns(ubscribe) some-project some-user
```

- added commands for listing currently subscribed user:

```
bender github subscribers # shows github subscribers for all projects
bender github subscribers some-project # shows github subscribers for some-project
```

- added possibility of registering own github / gitlab user with bender, so we don't get pull requests assigned to self

```
bender github me my-user
bender gitlab me my-user
bender github me # shows own username
bender gitlab me # shows own username
```

- when assigning pull / merge requests, bender now looks for subscribers; if none is found - falls back to initial behaviour (all collaborators)