Add instance_url to jira_connect_installations

💻 What does this MR do?

It adds a new column instance_url to the jira_connect_installations table. The column is needed for a feature that allows to use gitlab.com as a proxy for self managed instances using the GitLab for Jira App.

📺 More conext

I created a proof of concept for this feature and recorded this video to show how it works

🔗 Related issues

issue: #325280 (closed) epic: #325280 (closed)

💾 Database

Miragtion UP:

== 20210412132736 AddInstanceUrlToJiraConnectInstallations: migrating =========
-- add_column(:jira_connect_installations, :instance_url, :text)
   -> 0.0027s
== 20210412132736 AddInstanceUrlToJiraConnectInstallations: migrated (0.0027s) 

== 20210412132824 AddTextLimitToJiraConnectInstallationsInstanceUrl: migrating 
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0003s
-- execute("ALTER TABLE jira_connect_installations\nADD CONSTRAINT check_4c6abed669\nCHECK ( char_length(instance_url) <= 255 )\nNOT VALID;\n")
   -> 0.0013s
-- current_schema()
   -> 0.0002s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- execute("ALTER TABLE jira_connect_installations VALIDATE CONSTRAINT check_4c6abed669;")
   -> 0.0010s
-- execute("RESET ALL")
   -> 0.0006s
== 20210412132824 AddTextLimitToJiraConnectInstallationsInstanceUrl: migrated (0.0148s) 

Migration DOWN:

== 20210412132824 AddTextLimitToJiraConnectInstallationsInstanceUrl: reverting 
-- execute("ALTER TABLE jira_connect_installations\nDROP CONSTRAINT IF EXISTS check_4c6abed669\n")
   -> 0.0017s
== 20210412132824 AddTextLimitToJiraConnectInstallationsInstanceUrl: reverted (0.0135s) 

== 20210412132736 AddInstanceUrlToJiraConnectInstallations: reverting =========
-- remove_column(:jira_connect_installations, :instance_url)
   -> 0.0034s
== 20210412132736 AddInstanceUrlToJiraConnectInstallations: reverted (0.0034s) 

Does this MR meet the acceptance criteria?

📋 Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done