Revert throwing error when setting non-existing object subscribers
Summary:
Revert rPbabeed68 which can lead to an error when commenting on a task:
```
Subscriber transactions must be existing PHIDs or usernames (found key "0" on transaction of type "core:subscribers").
```
Looks like Phorge is pretty inconsistent in its array keys when passing subscriber arrays around - sometimes the keys are default integers, sometimes they duplicate the value.
Closes Q238
Reopens T16311
Test Plan:
* Comment on a task which you are not yet subscribed to via the web interface.
* Follow test plan of original rPbabeed68 for the Conduit part.
Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16311
Differential Revision: https://we.phorge.it/D26741