Replacing a global variable by a pseudo-global React context.

Let’s make it a React hook to allow caching the value.

Ideally, we would create the sharers once in EntriesPage and then pass them down
but that would break custom sharers since the `user.js` may take some time to load.

Requires attaching App after `selfoss.loggedin` is updated.

The jQuery removal was long coming.
Since the next version of Fancybox is unfree, let’s switch to something else.

It is bit more maintained and looks better.