Deal better with incoming tags

[akirk]

When a post with a hashtag is received, apparently WordPress automatically adds it as a tag, we need to deal with it better:

• Expose the tagged posts, it could be actually useful.
• Add a way to limit the tags, potentially to zero.

The first instict might be to just add a way to stop all incoming tags but maybe it is desired to keep a few of them, maybe of the topics you'd like to follow.

[akirk]

For what it's worth, I had explored this in #218 but never implemented it. Looks like WordPress does it on its own? I yet have to find evidence how it's done.

[akirk]

Turn of events: the ActivityPub plugin actually does inject them. Still it makes sense to deal with them better.