feat: Add custom link to collection

[suzubara]

Description

This PR adds the "Add link" flow to the Custom Collection component, allowing users to add arbitrary links to existing collections.

In the interest of keeping PRs small, this does not include the modal that asks a user to enter a custom label for their link. That feature is added in #250

I'm considering this branch to be feature complete since a user is still able to add a link. The display text will just default to the URL.

Fixes #233

Review Notes

• Verify you can click "Add link" on an existing collection in My Space to add a new URL to the collection.
• The text input should validate that you have entered a complete & valid URL. Entering other text or no text at all should prevent you from submitting the link.
• Since the link is only added in the local cache, refreshing the page will reset any changes you've made.

[github-actions]

🎉 View the deployed prototype - https://233-add-link-to-collection--ussf-portal-client.netlify.app

[abbyoung]

I was wondering if we need to create new _buttons and _global files outside of the mvp directory -- were we just using mvp for the rebuild? or keep it for now?

[suzubara]

These were just used for the mvp, until we start changing the global level branding/theming of standard USWDS buttons I think we can hold off

[abbyoung]

excellent! 🎸

[jbecker01]

👍