Plugins js updates

Updated **Plugins::load()** method and **blocks, menu, pages, snippets, users** plugins scripts for backend mode.
Removed **$.monstra** object is undefined conditional and **$.ready()** function from every plugin js file.
After update all plugins **init()** functions are running from one **$.ready()** function and fixed code is appending in **backend_site.minify.js**:
```javascript
function confirmDelete(msg){var data=confirm(msg+" ?"); return data;}

if (typeof $.monstra == 'undefined') $.monstra = {};
$.monstra.users = {

    init: function() {
        this.usersFrontendRegistration();
    },

    usersFrontendRegistration: function() {
        $("[name=users_frontend_registration]").click(function() {
            $("[name=users_frontend]").submit();
        }); 
    }

};
$.monstra.snippets = {

    init: function() { },

    showEmbedCodes: function(name) {
        $('#shortcode').html('{snippet get="'+name+'"}');
        $('#phpcode').html('&lt;?php echo Snippet::get("'+name+'"); ?&gt;');
        $('#embedCodes').modal();
    }

};
$.monstra.menu = {

    init: function() { },

    selectPage: function (slug, title) {
        $('input[name=menu_item_link]').val(slug);
        $('input[name=menu_item_name]').val(title);
        $('#selectPageModal').modal('hide');
    },

    selectCategory: function (name) {
        $('input[name=menu_item_category]').val(name);
        $('#selectCategoryModal').modal('hide');
    }

};
$.monstra.blocks = {

    init: function() { },

    showEmbedCodes: function(name) {
        $('#shortcode').html('{block get="'+name+'"}');
        $('#phpcode').html('&lt;?php echo Block::get("'+name+'"); ?&gt;');
        $('#embedCodes').modal();
    }

};
$.monstra.pages = {

    init: function() { 
        this.pagesExpandProcess();
    },

    pageExpand: function(slug, expand, token) {
        $.ajax({
            type:"post",        
            data:"slug="+slug+"&expand="+expand+"&token="+token,
            url: $('form input[name="url"]').val()
        });
    },

    pagesExpandProcess: function() {
        $(".parent").click(function() {
            if ($(this).html() == "-") {
                $('[rel="children_' + $(this).attr('rel')+'"]').hide();                                
                $(this).html("+");
                $.monstra.pages.pageExpand($(this).attr("rel"), "1", $(this).attr("token"));
            } else {
                $('[rel="children_' + $(this).attr('rel')+'"]').show();
                $(this).html("-");
                $.monstra.pages.pageExpand($(this).attr("rel"), "0", $(this).attr("token"));
            }
        });
    }

};
$(document).ready(function() {
    $.monstra.users.init();
    $.monstra.snippets.init();
    $.monstra.menu.init();
    $.monstra.blocks.init();
    $.monstra.pages.init();
});
```