GitLab for Jira app: add form for self-managed users to configure their GitLab instance URL

What does this MR do and why?

Part 2 of n for #356361 (closed)

⚠ this is behind the jira_connect_oauth_self_managed feature flag.

This MR adds the UI for self-managed sign-in to the Jira Connect app. Specifically, it adds a screen containing a form where a user can select if they would like to use dot-com or self-managed. If self-managed, they can enter their self-managed URL, which will be used to sign them in.

Part 1: !85483 (merged)

Screenshots or screen recordings

Kapture_2022-05-06_at_18.53.35

How to set up and validate locally

Follow the below steps to test the feature-flag-enabled state:

1. Set up Jira and the GitLab for Jira app: https://docs.gitlab.com/ee/development/integrations/jira_connect.html#install-the-app-in-jira
2. Enable the jira_connect_oauth and jira_connect_oauth_self_managed feature flags.
3. Refresh the Jira Connect app and observe. You should get a blank page (remember: the UI is coming in the next MR).

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.