Accessibility updates to copy buttons and textarea description

[mferrarini]

Description

An accessibility audit completed at Harvard revealed two issues that occur when screenreaders are interacting with the share plugin.

1. There are two buttons labeled "copy" that copy different info; When seen in context, the placement of these buttons are fine. Using a screenreader, though, when listing the buttons, there will be two buttons with the same text (copy) that copy different content, thus creating confusion.
2. textarea form field (where the embed code is offered for copying and pasting) is not labeled; every form field element should have a descriptive text label.

Fixes

1. Added aria-labels to the “Copy” buttons so that screen readers can differentiate the two options (copying a link vs. copying the iframe code)
2. Updated the "copy & paste code" element to be a label instead of a legend, so that screenreaders have a clearer description of the textarea form field when it is read aloud; this also involved updating the classname to be a label and giving it the same styles as it had before with the legend.

Unit tests were also updated to reflect the changes made related to the legend and label components.

[phil-plencner-hl]

I ran the plugin locally and I saw the changes to the labels and aria tags in the markup.
I also ran the unit tests and everything passed.