Replace `new_tag_field.vue` placeholder with real implementation

What does this MR do?

Replaces the placeholder content in new_tag_field.vue (added in !37598 (merged)) with a real implementation.

This is a step towards the New Release page being implemented as part of #214245 (closed).

This MR also extracts some duplicated markup into a simple wrapper component (form_field_container.vue) as suggested during a review of a previous MR: !37598 (comment 387058551).

What doesn't this MR do?

This MR does not complete the New Release page. This page is hidden behind a new_release_page feature flag and is currently disabled by default.

Screenshots

For additional screenshots and a GIF of these fields in action, see the description of !36466 (merged).

Testing locally

To test locally, enable the new_release_page feature flag and apply this patch: hack-to-test-locally.patch

git apply hack-to-test-locally.patch

This patch will makes some (terrible) edits that will allow you to navigate to <user name>/<project name>/-/releases/new and view the page without errors.

Note: This patch only allows the page to be rendered. Clicking the Save changes button will throw an error.

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Related to #214245 (closed)