Issue previews (unfurling) feature doesn't work if jirabot isn't invited by using /invite command on the Slack channel

Summary

When connecting a JIRA instance with a Slack channel using the /jira connect command, jirabot isn't added to the channel. This is causing issues with the issue preview feature. Providing the issue's URL or Key doesn't work, only using the /jira issueKey will pull the issue's information from the JIRA project.

Steps to Reproduce

1. Create a link between a JIRA project and a Slack channel by using the /jira connect command
2. Provide an issue's key or issue's URL on the Slack channel

Expected Results

The integration should pull the issue's information and display them on the Slack channel

Actual Results

Issue's information will not be displayed.

Workaround

The current workaround is to invite the jirabot explicitly by using the /invite jirabot command. After doing this, mentioning an issue's key or URL will pull the issue's information correctly.