Add Input Validation in DVCS Configuration Page for Gitlab/Github/GHE Option

Issue Summary

Gitlab integration configuration in Jira currently takes any string you type into the "Team or User Account" field without validating the configuration works.

Steps to Reproduce

1. Configure OAUTH 2.0 for Gitlab.
2. Navigate to Administration > DVCS Accounts and click "Link an account"
3. Toggle "Host" field to GitLab Self-Managed, or GitLab.
4. Type any random text you want into the "Team or User Account" field.
5. Save the configuration and observe it is successful.

Expected Results

Configuration should be validated prior to allowing you to save it to prevent invalid configuration.

Actual Results

The field takes whatever you type in.

Workaround

None.