Added more detailed strings to JIRA setup page.

I added some descriptive strings to the JIRA configuration page, to help clarify a few common issues that users encounter when setting up the JIRA integration.

Three common issues we are seeing are: (1) Users don't use the correct username, (2) JIRA projects don't have the "task" issue type enabled, (3) JIRA projects have required fields (which Code Climate can't populate).

The string changes in this PR address each of these issues.

@mrb and I discussed this.