[URL Drilldown] Range selection over date x-axis timestamp range instead of iso range

[Dosant]

Kibana version: master, 7.x
Describe the bug:

In 7.11 and prior when building a URL drilldown using range selection trigger would provide selection range in ISO format.
In master and 7.x this has changed to timestamps and this is a breaking change.

Steps to reproduce:

1. Create a dashboard with a vis with date histogram on the x-axis
2. Create a URL drilldown with Range selection. Use {{event.from}} {{event.to}} from the URL. For example, use to navigate to discover:

{{kibanaUrl}}/app/discover#/?_g=(filters:!(),refreshInterval:(pause:!t,value:0),time:(from:'{{event.from}}',to:'{{event.to}}'))&_a=(columns:!(_source),filters:{{rison context.panel.filters}},index:'{{context.panel.indexPatternId}}',interval:auto,query:(language:{{context.panel.query.language}},query:'{{context.panel.query.query}}'),sort:!())

Save the drilldown

3. Test the drilldown. See how {{event.from}} {{event.to}} substituted with timestamps instead of ISO strings like in 7.11 and prior. In the example URL case navigation to discover is incorrect because it doesn't support timestamps in time range.

Expected behavior:

{{event.from}} {{event.to}} substituted with ISO strings dates

Any additional context:

We refactored tabify which could have caused this changed.

There is a functional test that should have caught this regression:
https://github.com/elastic/kibana/blob/master/x-pack/test/functional/apps/dashboard/drilldowns/dashboard_to_url_drilldown.ts#L76

But because of not very strict assertion and the fact, the discover just swallowed not supported time range without any error, we missed the regression.

As part of the fix, we should also adjust that functional test.

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Please note: if we decide we want/must to with timestamps instead of ISO string, then there is a way to workaround example drilldown. Just using date helper in a template would force an ISO string: {{date event.from}}

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[ppisljar]

checked the tabify and things seem fine there, possible reason would be switching from vislib to xy_chart ?

[stratoula]

After syncing offline, we decided to:

1. Keep timestamps for URL drilldowns instead of ISO strings.
2. The only plugin that right now uses ISO strings is the vislib xy axis charts that we will change it to emit timestamps.
3. As URL drilldowns is in beta and introduced on 7.10 we won't migrate existing URL drilldowns.