Add unique constraint for column activity.ap_id

[Nutomic]

Then remove function is_activity_already_known(). No need to make a separate query and check in Rust, when we can simply rely on db constraints. The migration needs to delete any existing, duplicate values first.

[dessalines]

I think this already exists:

Indexes:
    "activity_pkey" PRIMARY KEY, btree (id)
    "idx_activity_unique_apid" UNIQUE, btree ((data ->> 'id'::text))

[Nutomic]

I tried and was able to insert duplicate values without any problem.

Btw, another thing I would like to do is change community.public_key into String (instead of Option<String>). Same for person.public_key.