Allow modification of search-box placeholder

[RFSH]

Currently, we allow modification of search columns through the search-box property under search-definitions. To inform users about the modified behavior of search box, we're changing the placeholder to show a comma-separated list of displaynames.

As you can imagine, this can become very lengthy, and also, listing every column might not be helpful to users.

So as was proposed before, we should allow data modelers to define their own placeholder_markdown.

"tag:isrd.isi.edu,2019:source-definitions": {
  "sources": {
    ...
    "search-box": {
      "or": [...],
      "placeholder_markdown": "Shorter name for placeholder text in input",
    }
  }
}

As part of this, we should also add the computed placeholder (default value or the one customized with this annotation) to the first line of the tooltip for the search button. This is because the placeholder will disappear when you have a text in the search box, and we want users to be able to access it.

P.S. The original issue discussed adding a comment property, but we decided not to add it. We agreed that we don't want the tooltip we display when the content is long to differ from the content itself. We also didn't think allowing data modelers to modify the tooltip of the search button was needed.