show forum subscriptions in user mangement + allow mods to subscribe users to threads arbitrarily

[donrestarone]

there is a button on each forum thread that allows the current user to subscribe / unsubscribe to a thread. We want to:

1. List all the threads a user is subscribed to in this view (nest it under a button, Forum):

2. on a specific thread, add a UI + server logic that allows admins to search for users and subscribe them to the specific thread (if they are not already subscribed, and allow subscriptions to be removed as well)

tldr; admins should be able to unsubscribe/subscribe users to forum threads