SearchKit - Add default text/image to displays; support custom file fields

[colemanw]

Overview

Allows for "placeholder" text in a search display when a column value is empty. Also works for image fields.
Image fields now support custom file fields to display images out-of-the-box.

Technical Details

This breaks out image into its own column type, and generates the url to access custom file fields.
A new 'empty_value' property allows for a placeholder text/image to display when a field is blank.

[civibot]

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[colemanw]

retest this please

[jaapjansma]

test this please

[jaapjansma]

We (@kainuk, @BettyDolfing and I) are reviewing PRs and we checked this PR.

• General standards
  • Explain (r-explain)
    • PASS : The goal/problem/solution have been adequately explained in the PR. This could have been explained better it tooks some guesses and what to do. We missed a step a step by instruction on how to test or reproduce this.
  • User impact (r-user)
    • PASS: The change would noticeably impact the user-experience (eg requiring retraining), however we believe it is a naturally approach.
  • Documentation (r-doc)
    • FAIL: The user documentation should be updated. E.g. https://docs.civicrm.org/user/en/latest/the-user-interface/search-kit/#search-displays
  • Run it (r-run)
    • PASS: We created a new search with search kit and included a custom file field and displayed this as an image. We also provided a default image for when the field was empty. We then checked the output of the display and it all seemed to work.
• Developer standards
  • Technical impact (r-tech)
    • PASS: The change potentially affects compatibility, but the risks have been sufficiently managed.
  • Code quality (r-code)
    • PASS: The functionality, purpose, and style of the code seems clear+sensible.
  • Maintainability (r-maint)
    • PASS: The change sufficiently improves test coverage.
  • Test results (r-test)
    • PASS: The test results are all-clear.

@colemanw can you also update the user documentation?

Although we miss the update for the user documentation we believe this PR is ready to be merged. @demeritcowboy what do you think?

[colemanw]

Thanks for the review @jaapjansma!
I will try to write some docs when I get time, but right now I'm focused on building out features for SearchKit.
It would be nice to organize a docs sprint just for SearchKit now that people are really using it.