ENH: Pin and Like subscriptions

"Pinning" a topic and/or "liking" a post means that the content is important to the community and contains useful information. You can currently subscribe to either a forum (which means you get notifications for *ALL* topics and replies), or a specific topic (meaning ANY replies to that topic). This new subscription option would be a middle option, sending you notifications only for "pinned" topics in a forum and/or "liked" posts. You would receive a notification when a moderator pins a topic, and any replies are made to the topic. For likes, any time a post is "liked" (or perhaps on first like), you get a notification.

Start by reviewing the existing forum and topic subscription flows, then trace how moderators pin topics and users like posts. Clarify whether notifications occur on every like or only the first, define the subscription settings and notification behavior, and identify the relevant implementation and tests before estimating the work.

Written by the indexing model from the issue text.