Expiry date on pinned nav cookie

## What does this MR do?

Adds an expiry date far into the future for the pinned nav cookie so that it survives logout & browser closing.

See merge request !5009

Global Ajax error handler

## What does this MR do?
Introduces a new global Ajax error handler to show a flash warning about the error detail of XHR requests which failed with status code 404 and 500

## Are there points in the code the reviewer needs to double check?
Double check the wording.

## Why was this MR needed?
A global solution for the problem in the #17880

## What are the relevant issue numbers?
Fixes #17880 

## Screenshots (if relevant)

For requests failed with 404 - 500 

![Screen_Shot_2016-06-21_at_00.07.26](/uploads/04bb3a5a958c3d21aef4fd12bd3d105a/Screen_Shot_2016-06-21_at_00.07.26.png)

For requests failed with 401

![Screen_Shot_2016-06-21_at_00.08.30](/uploads/25cc885ba388f4c6f4c555fa338e1a63/Screen_Shot_2016-06-21_at_00.08.30.png)

See merge request !4797

This makes larger libraries more cacheable and will allow us to use SRI with the dynamically included libraries.

jQuery selector refactor in application.js.

Minor refactor for unnecessarily repeated `$(document)` `$(window)` and `$('body')`

See merge request !4809