Update New Project page dropdown and QA tests

What does this MR do and why?

This MR updates the New Project page dropdown and QA tests. The QA tests are being updated to pass successfully with the feature flag paginatable_namespace_drop_down_for_project_creation switched on because we plan to switch on this feature flag on staging soon.

New Project page dropdown changes

Two changes were made to the dropdown:

1. The dropdown value is defaulted to the user's namespace if a namespace_id URL parameter is not provided
2. The dropdown width is restrained so that when a long value is selected it does not stretch out onto the other input field to the right (see screenshots below)

QA test changes

The Project URL select2 dropdown was recently replaced by a GlDropdown Vue component in !68778 (merged), behind the feature flag paginatable_namespace_drop_down_for_project_creation.

• The Project URL dropdown was a select2 dropdown in app/views/projects/_new_project_fields.html.haml
• The Project URL dropdown was converted to a GlDropdown dropdown in app/assets/javascripts/pages/projects/new/components/new_project_url_select.vue

Issue: #26732 (closed)

Screenshots or screen recordings

before	after

Selecting to use a group template (filters the dropdown to only the selected group and children)

Screen_Recording_2021-10-07_at_2.32.51_pm

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:paginatable_namespace_drop_down_for_project_creation)

2. Visit the new project page such as http://127.0.0.1:3000/projects/new

3. Test the changes

MR 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 MR acceptance checklist for this MR.

Related to #26732 (closed)