Time filter from older versions break exploring slices from dashboard

[octaviancorlade]

Upgrading from 0.26.3 to the latest master branch, using a dashboard with "from" and "to" filters stored as dashboard metadata in the previous version, like

"default_filters": "{\"1234\": {\"__from\": \"15 days ago\", \"__to\": \"1 days ago\"}}"

Following #4981 these are not converted to the new __time_range filter and not being treated as date options in utils.merge_extra_filters they are used as filters instead:

"{"expressionType":"SIMPLE","subject":"__from","operator":"in","comparator":"15 days ago","clause":"WHERE","sqlExpression":null,"fromFormData":true,"filterOptionName":"63ad1398-eec0-449f-b0ae-7b8ab2c77b21"}"

Since the operator is "in" and the comparator is not an array, it also breaks when exploring any slice from that dashboard:

webpack:///./src/explore/AdhocFilter.js?:48 Uncaught TypeError: adhocMetric.comparator.join is not a function

[stale]

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