New project subgroup/searchable dropdown

[malchua]

What does this PR do and why?

Describe in detail what your merge request does and why.
This PR adds the searchable dropdown to the new Project and new Subgroup forms. This dropdown will make it easier for a user to look up a specific Namespace or Group when creating a new Project or Subgroup.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other pull requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Login
2. In the sidebar, click on Groups
3. Select a group that your are an owner of
4. On the details page, click on the New project button
5. In the Project URL, search for any Namespaces that you have at least the Maintainer role for and check that they present on the list.
6. Repeat steps 1-5 with the New subgroup button and Group URL section.

PR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the PR acceptance checklist for this PR.

[github-actions]

Code Metrics Report

Coverage	Test Execution Time
93.3%	8m48s

Code coverage of files in pull request scope (100.0%)

Files	Coverage
app/views/groups/_form_fields.html.erb	100.0%
app/views/groups/new_subgroup.html.erb	100.0%
app/views/projects/_project_namespace_fields.erb	100.0%
app/views/projects/new.html.erb	100.0%

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Reported by octocov

[joshsadam]

Just took a quick look through this. My one major comment I have is that if I were a user and I clicked "Create project" from within a group, I would expect that the namespace would auto-populate to be that namespace and I would not need to search for it, possibly even disable changing it.

[joshsadam]

nitpick: Can you see if there is a way that you can get the left corners of the select to be 90 degrees so they stay inline with the root URL corners:

[joshsadam]

Looking through LookBook, I actually created a viral component that you can use to do exactly this http://localhost:3000/rails/lookbook/inspect/viral_form_prefixed_select/default