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.

@noahd1 Please merge this one after you review it.
I'm reopening after reading the text a little more closely. The last sentence is a bit hard to read -- can we find a simpler way to say it? It has "must" twice and the "must not have non-standard" feels like a double-negative. Does it have the same meaning if we flip it? "must only have standard required fields" ?

> Project must support 'task' issue types and must not have non-standard required fields.