Add widget instruction link to custom alt text guidelines in Exhibits

[alundgard]

The “Guidelines for writing alt text” link opens the guidelines in a new browser tab.
Add customizability for institution-specific instructions, specified per Spotlight instance. For Exhibits, add Stanford’s alt text instructions: https://uit.stanford.edu/accessibility/concepts/images

[marlo-longley]

Blocked by #2582 -- the feature first needs to be implemented in Spotlight, then overridden in Exhibits here.

[hudajkhan]

I have set up this PR: projectblacklight/spotlight#3188 in Spotlight to address #2582.
If the current method is approved, overriding the link would look like the following in sir_trevor_block_overrides:
SirTrevor.Locales.en.blocks.alt_text_guidelines = $.extend(SirTrevor.Locales.en.blocks.alt_text_guidelines, {
link_url: 'http://www.google.com'
});