[Stats Refresh] Period Referrers: fix displaying details view

[ScoutHarris]

Ref #11360, #11080

This changes Period > Referrers to use UITableViewCells instead of one UIStackView.

Previously, all the data was stuffed into the stack view on the TopTotalsCell card. Now it only uses the TopTotalsCell card to show the header via DetailSubtitlesHeaderRow. The stack view is left empty. Instead, a UITableViewCell is added to the table for each data row via:

• DetailExpandableRow - for parent rows
• DetailExpandableChildRow - for child rows

And, because Referrers Search Engines rows can be expanded twice, "grandchild" rows are now supported.

This also addresses two other issues:
From #11080 - When no icon is available then show gridicons-globe.
From #11674 (comment) - fix gridicons-search icon size.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

To test:

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• Go to Stats > Period > Referrers.
• For any Referrers that don't have an icon, verify the globe icon is displayed.

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• Select View more.
• Verify the data appears relatively quickly.
• For any Referrers that don't have an icon, verify the globe icon is displayed.
• For a non-expandable row, verify row selection shows a web view.

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• For an expandable row, row selection shows all child rows.
• When a row is expanded:
  • The child label text color is light grey.
  • Child rows have no separator line between them.
  • There is a full width separator line above the parent row, and below the last child.
• Selecting a child row shows a web view.

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• For the Search Engines row: (tip: en.blog has this row)
  • Row selection shows all child rows.
  • If a child row is expandable, it looks like a parent row, i.e. text color is dark grey, and the chevron is pointed down.
  • When a child row is expanded, it exhibits the same behavior has listed above (For an expandable row...).

[ghost]

I know this code was in place previously, but it occurred to me that some might not understand the intent of this message.

If we're making the image view circular, it might be more clear to set the path for a mask via one of the following methods:

• init(arcCenter:radius:startAngle:endAngle:clockwise:)
• init(ovalIn:)

I don't feel strongly about this, but wanted to share.

[ghost]

@ScoutHarris this is working as described for me!