Convert country targeting to tree-based checkboxes

[coderberry]

Replace the existing country selection with a tree-based version.

As an admin,

• if I select a subregion, it selects all countries in the subregion
• if I select a subregion, I can deselect countries within the subregion

[gitcoinbot]

Issue Status: 1. Open 2. Started 3. Submitted 4. Done

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This issue now has a funding of 2.14 ETH (500.08 USD @ $233.68/ETH) attached to it as part of the gitcoinco fund.

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[gitcoinbot]

Issue Status: 1. Open 2. Cancelled

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Work has been started.

These users each claimed they can complete the work by 1 week from now.
Please review their action plans below:

1) brascoder has been approved to start work.

Update markup to use a checkbox tree
Add stimulus controllers to manage the tree and the branches

Learn more on the Gitcoin Issue Details page.

[gitcoinbot]

Issue Status: 1. Open 2. Cancelled

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The funding of 2.14 ETH (519.7 USD @ $242.85/ETH) attached to this issue has been cancelled by the bounty submitter

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[gitcoinbot]

⚡️ A tip worth 1.97930 ETH (499.99 USD @ $252.61/ETH) has been granted to @brascoder for this issue from @coderberry. ⚡️

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