Add method to remove entries from a channel

[aaronpk]

Users may want to remove an entry from a channel for various reasons. Since #4 tracking read state means we now have referenceable entry IDs, we can use those to also remove entries from a channel.

Here is what I've implemented in Aperture:

POST /microsub

action=timeline
&method=remove
&entry=X

• method=remove vs delete because the server may not actually delete the entry, for example if the feed is added to another channel, or to allow undos
• entry or entry[] - may be a single value or multiple values

[aaronpk]

This is documented here: https://indieweb.org/Microsub-spec#Remove_Entry_from_a_Channel

[swentel]

Drupal IndieWeb and Indigenous for Android support this.