[VUFIND-747] Create Breadcrumbs view helper.

[demiankatz]

This PR adds a view helper that uses templates to render breadcrumb trails. This makes it easier to customize and modify breadcrumbs globally, and makes breadcrumb construction more readable in individual templates.

For now, the helper manipulates $this->layout()->breadcrumbs for backward compatibility, but it could be modified to use its own internal string instead of the layout at some point in the future if we want to make it more self-contained.

TODO

• Finish porting templates to use the helper
• Run full test suite
• Update v12 deprecation ticket to note RecordLinker::getBreadcrumbHtml deprecation
• Update changelog when merging
• Resolve VUFIND-747 when merging

[demiankatz]

Thanks, @sturkel89 -- I have resolved the conflict here and have also reviewed your latest updated templates. They all look correct, but I had a few minor stylistic comments. You can use the buttons in my comments below to apply the changes and save yourself the trouble of making the edits yourself, and then you can pull the latest down to your environment when you're ready to work on more.

[demiankatz]

All tests are now passing!

[EreMaijala]

With the unused translations @demiankatz mentioned removed, this looks good to me!

[demiankatz]

Thanks, @EreMaijala, I cleaned up the language files and added some unit tests before merging this.