[minor] Fix "get link" detail navbar feature

[abought]

Purpose

The default templates have a feature that doesn't render as written. The copy button is only shown when https_url is defined, but the field is filled in based on a (never-defined) variable called copy_to_clipboard_link.

Short term fix

This is the narrowest possible fix, just to avoid that moment of puzzlement for any new user.

Use the same variable to render the copy widget that was used to render the button.

Reduces tiny risk of breakage by changing the URL variable at point of usage, to match what the page already assumed it was called in detail-nav.html block detail_nav_get_link_button.

Left the nominally reusable partial unchanged. (copy-to-clipboard.html).

Long term

Most people are probably sharing datasets (not single files). The current implementation is not super aligned with user needs, which may explain why no one noticed. This is the smallest possible fix to restore stock functionality.

Example

Before

After

[NickolausDS]

Could we keep the v2 templates the same, and remove copy_to_clipboard_link entirely from the v3 templates? v3 is not currently the stable default, so changes like these are encouraged there.

[abought]

Given that no one ever noticed and reported this in 4 yr, I suspect there's a lot of latitude. USGS (primary known v3 users) indicate that they're doing a full override.

To clarify:
"Keep the same" = keep the change in this PR, or "replace this commit with an entirely different approach for both templates"?

[NickolausDS]

Remove the change in v2, in case some portal out there does make use of these two items being different.

In v3, remove copy_to_clipboard_link entirely in favor of https_url. We could use your implementation instead if we want to support this snippet as something modular which could be used in other contexts, such as a list of files with copy-able links. But supporting that would also require additional work, since id=collapseExample makes that not currently possible with this template. Doing away with the additional copy_to_clipboard_link term seems more appropriate for simplicity.