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

Originally discussed in the parent epic, particularly &5650 (comment 847058069).

Set up steps for self-managed users

1. User chooses GitLab version (.com vs. self-managed)
2. User chooses self-managed (At this point, should we provide additional information around how to create the System OAuth application?)
3. User is prompted to input self-managed instance URL
4. User is prompted to sign in to GitLab (would this be .com or self-managed or both?)

step 1	step 4

Remaining effort

Remaining effort for %15.1: frontend-weight1