Improvements for the sidebar toggle button

Problem/Motivation

The sidebar toggle button has the "is-active" class when the sidebar is visible; this class is used elsewhere for active elements of a given set and so it makes no sense here, grabbing extra focus from the user unnecessarily.

It also has a title attribute that is redundant, given the existence of the same text in the tooltip.

Sometimes, it receives a click event in cases where it shouldn't, toggling the sidebar without the user's intent.

Steps to reproduce

For the unintended click I don't have clear steps to provide, but it happens sometimes right after switching back to the browser tab from another tab. The gin_toolbar & admin_toolbar modules might or might not be involved in this, with or without the use of Ctrl+click on various admin menu links.

Proposed resolution

Replace the "is-active" class with a "can-collapse" class & remove the title attribute (see attached image), and fix the unintended button activation.