Feature Suggestion: Add filter hints to filter tooltip popover

[redbassett]

Using administrate on a client project recently, I was asked if it was possible to add clarifying text to describe what filters within the search for a dashboard did. I have brought the solution I implemented within that project to the core administrate codebase in this PR as a feature suggestion if this is useful for other users of administrate.

Developers using administrate can add a filter_hint string translation, similar to the field_hint translation used for form hints, and the value of the translation will be displayed in the filter tooltip popover shown when filters are present in a dashboard:

en:
  administrate:
    filter_hints:
      customer:
        inactive: "Only show customers who haven't logged in in the past week"

By default the styling of the popover handles this additional text without any major issues, however long strings can stretch this popover significantly. I have included bare minimum styling to pad the hint text away from the filter key/value text for readability but I haven't addressed long hint strings with this styling as I didn't want to change the existing popover styles too much. Such styling can be added if this is considered to be a likely issue.

This PR includes the view logic to find filter hints, adds a filter hint to the example app to demonstrate the functionality, adds documentation within the relevant section on filters, and adds test coverage modeled on field_hint tests.