Add ability to pin posts

We support post pinning in the BE, but no way to handle it in the FE

https://docs.joinmastodon.org/api/rest/statuses/#post-api-v1-statuses-id-pin

assigned to @Dave

mentioned in merge request !731 (closed)

added mastoapi label

mentioned in merge request !739 (closed)

assigned to @feld and unassigned @Dave

assigned to @tae and unassigned @feld

mentioned in merge request !770 (merged)

mentioned in commit fedcc716

closed via merge request !770 (merged)