Migrate enable Gitpod confirmation modal to `GlModal`

What does this MR do and why?

Related to #344567 (closed)

When viewing a repository tree, file blob, or MR there is an Gitpod button. If the user does not have Gitpod integration enabled yet it will display a modal asking if the user wants to enable the Gitpod integration. Currently this is a Bootstrap modal. This MR migrates that modal to GitLab UI GlModal.

Screenshots or screen recordings

MR

Before	After
Screen_Recording_2022-01-10_at_4.10.04_PM	Untitled__2_

File blob

Before	After
Screen_Recording_2022-01-10_at_4.22.41_PM	Untitled__3_

Repository tree

Before	After
Screen_Recording_2022-01-10_at_4.23.27_PM	Screen_Recording_2022-01-10_at_4.33.30_PM

How to set up and validate locally

1. Ensure :refactor_blob_viewer feature flag is disabled
   • The refactored blob view does not yet use the Gitpod button
2. Sign in as an Admin
3. Navigate to /admin/application_settings/general#js-gitpod-settings and enable Gitpod
4. Navigate to /-/profile/preferences and ensure Enable Gitpod integration is unchecked
5. Navigate to a Merge Request and click Open in Gitpod
6. Navigate to a Repository and click Gitpod from the top toolbar
7. Navigate to a file in a repository and click Gitpod from the top toolbar

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.

locale/gitlab.pot

@@ -37134,7 +37134,7 @@ msgstr ""
 msgid "To update Snippets with multiple files, you must use the `files` parameter"
 msgstr ""
-msgid "To use Gitpod you must first enable the feature in the integrations section of your %{user_prefs}."
+msgid "To use Gitpod you must first enable the feature in the integrations section of your %{linkStart}user preferences%{linkEnd}."
 msgstr ""
 msgid "To use the additional formats, you must start the required %{container_link_start}companion containers%{container_link_end}."
@@ -43082,9 +43082,6 @@ msgstr ""
 msgid "user namespace cannot be the parent of another namespace"
 msgstr ""
-msgid "user preferences"
-msgstr ""
-
 msgid "username"
 msgstr ""