adding ability to disable a pfe-collapse-toggle button

[kylebuch8]

Fixes #516

adding ability to disable a pfe-collapse-toggle button

What has changed and why

Summarize files edited as part of this MR along with a brief description of what was changed/why.

• added the ability to add a disabled attribute to pfe-collapse-toggle to prevent it from opening a panel

Testing instructions

Be sure to include detailed instructions on how your update can be tested by another developer.

1. Visit the demo page and go to the last example at the bottom of the page. You should not be able to expand the panel.

[mwcz]

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