Prior to this commit, to share an entry, a user has to click on the
share link and then copy the URL they are redirected to. The danger is
that they may right-click and copy the share link without actually
clicking on it, and therefore share a link that, when authenticated,
shares the entry, rather than actually sharing the entry.
Here, we avoid this misinterpretation by making sharing into a POST
request and using a form rather than a link.
Currently, removing a feed from `/category/{id}/feeds` redirects incorrectly to `/feeds`. This change fixes it so that
removing a feed will now correctly redirect to `/category/{id}/feeds`. Removing a feed from `/feeds` is unaffected and
will work as it does currently.
To fix this, a new UI endpoint `/category/{categoryID}/feed/{feedID}/remove` is added and a corresponding handler method
to validate and perform the removal from DB.
