* Fix click through behavior (#15014)

* formatting

* Safer get's, author metadata and correct added

* Don't return anything if we didn't modify it

* Make private and add a docstring

* Generate json strings rather than long in-line versions

* Docstrings and prevent migrating ones that may already be migrated

* Noisier as it now merges click_behavior on top of existing

* update docstring with new nesting

* always keep column settings

previously the script was only looking for column settings which
included view_as or link_template. Now we always keep those columns
and merge in the new click_behavior when those keys are present

* linkTemplateText -> linkTextTemplate

* Ensure dashcard goes on top of card viz settings

* Update expected now that dash merges on top of card

* drop columns that don't have view_as

so we don't overwrite the top level stuff if there is already a
click_behavior. That signifies that either this migration has run, or
perhaps already has had manual intervention. An open question is if we
extend this to the columns: if there's manual intervention do we
really want to be mucking about in here?

Also, go back to the dropping columns style.

```clojure
(if (and ...)
    (assoc m col (merge field-settings click-behavior))
    m) ;; here we omit the column again in our reduce-kv
```

* Fundamentally: scour card for click and merge them into dashcard

reshape all click behaviors in both and merge them in.

Still some broken tests at the moment

* Remove old style click behavior no longer expected

* fixed

* Some more comments and remove some silly select keys and when-lets

just compute both keys and then u/select-non-nil-keys and be done with
it

* Docstrinc additions about nil

* Remove niles, test scenario #5 from flamber

* Handle empties and nil _AFTER_ merging

previously had taken care to punt things down to nil, collapse empty
maps, etc. but this is a value that will merge on top of a map rather
than with the map

```clojure
(m/deep-merge {:a {:b :c}} {:a nil})
{:a nil}
;; vs
(m/deep-merge {:a {:b :c}} {:a {}})
{:a {:b :c}}
```

so let the postwalk after the merge clean up empty collections and nil
values and let them blend as normal

* Deep merge and prioritize new shape in card and dashcard

```clojure
(m/deep-merge fixed-card
              fixed-dashcard
              (existing-fixed card)
              (existing-fixed dashcard))
```

This is the change here essentially. Fix the card and dashcard, look
for existing new shapes on the card and dashcard as well

* Don't merge existing fix from card. can't happen

this change though did help out. we want to resolve when there are old
style and new style on the same dashcard. In this case, the new style
should persist:

```clojure
(testing "If there is migration eligible on dash but also new style on dash, new style wins"
  (let [dash {"column_settings"
              {"[\"ref\",[\"field-id\",4]]"
               {"view_as"       "link"
                "link_template" "http://old" ;; this stuff could be migrated
                "link_text"     "old"
                "column_title"  "column title"
                "click_behavior"
                {"type" "link",
                 "linkType" "url", ;; but there is already a new style and it wins
                 "linkTemplate" "http://new",
                 "linkTextTemplate" "new"}}}}]
    (is (= {"column_settings"
            {"[\"ref\",[\"field-id\",4]]"
             {"click_behavior"
              {"type" "link",
               "linkType" "url",
               "linkTemplate" "http://new",
               "linkTextTemplate" "new"},
              "column_title" "column title"}}}
           (migrate nil dash)))))
```

note the column settings for field-id 4 have the old style and the new
style but the result is that the new style survives. This happens from
the `(m/deep-merge fixed-card fixed-dashcard (existing-fixed
dashcard))` as the existing is put on top

* Don't strip original values

* propagate old info in migration test