Responsive Navigation items

[ferishili]

This PR fixes #868,

Description

please refer to the issue's description,

Solution

• In order for Stud.IP to render the navigation of plugins in responsive, it is required to pass them as navigation subs in getTabNavigation via addSubNavigation.
• In order for Stud.IP to render the sidebar items drawer for the plugin, it is required to add sidebars in the php side via Sidebar class -> addWidget method.
• In our case, that we use CourseSidebar.vue to render everything in one place, we had to delete the Stud.IP rendered sidebar widget.
• To make sure that external links perform and load required data, we had to have a more accurate procedure to handle Views in CourseSidebar.vue -> handleView
• In Responsive view: in order to toggle the sidebar upon clicks, we are using pure js (exception => there are no other way)

Extra

• I saw that Opencast Plugin still uses video2 icon, although from StudIP version 5, there is an Opencast icon. Therefore, I have replaced that video2 icon with opencast icon!

To test

• Make sure you have this PR patched.
• in plugin's root directory perform npm install && npm run zip
• Use Responsive Mobile view in your browser or simply use a cell phone to test the plugin, using the navigation tabs etc. to go back and forth and perform every actions within the plugin (login with test_dozent is recommended)