Add gallery_api/annotations?ns=foo endpoint

[will-moore]

Fixes #57

This is an investigation into how to hide Projects and Screens until we want to publish them in IDR:

Steps:

• $ ssh -A -o 'ProxyCommand ssh idr-testing.openmicroscopy.org -W %h:%p' omeroreadwrite -L 1080:localhost:80 then login as demo user at local http://localhost:1080/webclient and add e.g. a Comment to Project/Screen.
• You can see the Comment ID using the browser Dev tools Network tab when re-loading right panel.
• Then, use CLI to set the namespace on that Comment.

$ omero obj update CommentAnnotation:38359142 ns=idr.gallery.hide

We need to inject the following script into main webclient page:
What it does:

• When the page loads, it makes an immediate request to the endpoint in this PR to load annotations with ns=idr.gallery.hide and the IDs of attached Screens and Projects.
• It then listens for load_node.jstree events on the jsTree and filters loaded child nodes to exclude the Screens and Projects that we want to hide

<script>
        $(document).ready(function(){
            let projectsToHide = [];
            let screensToHide = [];
            fetch("/gallery_api/annotations/?ns=idr.gallery.hide&type=project&type=screen")
            .then(response => response.json())
            .then(data => {
                data.annotations.forEach(annotation => {
                    console.log("ANNOTATION", annotation);
                    let link = annotation.link;
                    if (link.parent.class === "ProjectI") {
                        projectsToHide.push(link.parent.id);
                    } else if (link.parent.class === "ScreenI") {
                        screensToHide.push(link.parent.id);
                    }
                });
            });
            $("#dataTree").on("load_node.jstree", function(event, data) {
                data.node.children = data.node.children.filter(function(childId){
                    let child = data.instance.get_node(childId);
                    // console.log(child.type === "project", projectsToHide.includes(child.data.id));
                    if ((child.type === "project") && projectsToHide.includes(child.data.id)) {
                        console.log("Hiding project", child.data.id);
                        return false;
                    } else if ((child.type === "screen") && screensToHide.includes(child.data.id)) {
                        console.log("Hiding screen", child.data.id);
                        return false;
                    }
                    return true;
                });
            });
        });
    </script>