adding guidelines for alt text to specific sir trevor blocks

[hudajkhan]

Closes #3187 and sul-dlss/exhibits#2582.

What this pull request does:

• Add the guidelines intro, link label, and url to the locales file for Sir Trevor Blocks.
• Display these guidelines in widgets where alt text can be added: various versions of item views (fueled by a base solr document block) such as item row, item carousel, item grid, and item slideshow, and the uploaded items block.

[taylor-steve]

@hudajkhan I tried this, and it works well. What do you think about adding tests, maybe piggybacking on/adjacent to the alt text feature tests from https://github.com/projectblacklight/spotlight/pull/3180/files? Literally only checking for existence/non-existence of the link. Worth it?

[hudajkhan]

Thank you for helping me out @taylor-steve . I have added some tests.