Fix admin sidebar link a11y issues

[YanaDePauw]

References

Fixes #1170

Description

This PR ensures that users can tab through the sidebar menu using the keyboard.
Currently the expandable menus are still separated into two parts where the first is the icon, and the second is the link itself.
Tabbing through the menu will only pass over the icons. The text next to icon will be ignored to avoid tabbing to the same link twice.

The issue of separation of the icon and text will be addressed as part of #1185.

Instructions for Reviewers

• Log in as an administrator
• Verify that you can tab over all the entries in the side bar menu
• Verify that pressing enter on a collapsed menu will open it and that you can navigate through the submenu using tab

Checklist

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• My PR passes TSLint validation using yarn run lint
• My PR doesn't introduce circular dependencies
• My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• If my PR includes new, third-party dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.

[tdonohue]

👍 Thanks @YanaDePauw , this looks correct to me, and looks to solve the admin sidebar issues as described by Deque.