[db migration] add parent ids into Dashboard layout metadata

[graceguo-supercat]

This migration is to add parent ids for each UI element in dashboard layout metadata. This will be used by another feature (#6964): to provide a redirect link that can open dashboard in any tab.

Expected output:

We store dashboard layout metadata in a flatted tree structure. In old layout metadata, we only store children node for each UI component. This information is good enough for render dashboard from root element, and we always render first tab when dashboard opens. But If user want to see some information in other tab (or any chart in a nested tab), it will takes a few clicks to navigate to the point of interest.

This problem can be resolved if each UI component carries its parent node information. When we have parents list for each UI component, it's easy from dashboard root to render each parent id, and at last render the point of interest chart/tab.

Because this db migration is independent of UI change (do not block dashboard display), I will send out UI change in a different PR.

@john-bodley @michellethomas @kristw @williaster

[codecov-io]

Codecov Report

Merging #6945 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6945   +/-   ##
=======================================
  Coverage   64.63%   64.63%           
=======================================
  Files         422      422           
  Lines       20595    20595           
  Branches     2253     2253           
=======================================
  Hits        13311    13311           
  Misses       7161     7161           
  Partials      123      123

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