Render Jira app views based on key storage enabled

What does this MR do and why?

This fixes a bug that rendered a misleading view for self-managed users.

There are two ways to install the GitLab for Jira app. If installed from the official marketplace listing, the app renders the subscriptions page from GitLab.com. On this page users can choose if they want to use GitLab.com or a self-managed instance.

The other way is to manually install the app from a self-managed installation. In this case, the subscriptions page gets rendered directly from the self-managed instance, so there should be no option to use GitLab.com. However, after default enabling and removing the jira_connect_oauth_self_managed feature, users see the GitLab.com option on their self-managed installations. The page should actually look like this:

Note: The Sign in to GitLab.com text is still misleading but this can be fixed in a follow-up.

In this MR we render the page based on the jira_connect.enable_public_keys_storage config. Which is only default enabled for GitLab.com

How to set up and validate locally

1. Follow the simple setup guide to set up a Jira app development environment
2. Disable the enable_public_keys_storage config in config/gitlab.yml
3. Reload the page to not see the GitLab.com option anymore.

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 #388943 (closed)