Replace "Add Card" tab with sub-tab "Edit Cards"

[ryanml]

This is a sub issue under: #9616
Specification: https://docs.google.com/document/d/14lWBt0QmHDfA2G7h0hUPxZ8mVUd008IZPQJJfRryC9U/edit#

(Note, this does not implement the full specification with notifications, only the "Edit Card" addition.

Test Plan

1. Open up the NTP from a fresh profile
2. Confirm that "Edit Cards" appears under the widget stack
3. Confirm that clicking "Edit Cards" brings up the NTP settings focused on the Cards section
4. Confirm that when all of the widgets are hidden, "Edit Cards" disappears
5. Confirm that once a widget is added back from zero, "Edit Cards" re-appears

[srirambv]

Verification passed on

Brave	1.19.72 Chromium: 87.0.4280.101 (Official Build) beta (64-bit)
Revision	9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS	Windows 10 OS Version 2009 (Build 19042.685)

• Verified test plan from issue description
  

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Verification passed on

Brave	1.19.72 Chromium: 87.0.4280.101 (Official Build) beta (64-bit)
Revision	9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS	Linux

• Verified test plan from issue description
  

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Verification passed on

Brave	1.19.72 Chromium: 87.0.4280.101 (Official Build) beta (x86_64)
Revision	9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS	macOS Version 10.15.7 (Build 19H2)

• Verified test plan from issue description