Address two inconsistencies where general comments were not being handled in

the same fashion as the other classes of comments (e.g. diff, file attachment,

and screenshot comments).  These would lead to errant details in review emails 

for reviews where a Ship It had been declared.

The existence of general comments was not being considered when determining

the ship_it_only state of Review objects.  This resulted in review emails

where the X-ReviewBoard-ShipIt-Only header was set even when general comments

were present.
The existence of general comments with open issues was not being considered

when determining the need for the "Fix it, then Ship it!" designation for review

emails.  This produced review emails with the "Ship it!" designation, even when

the review contained general comments with issues.

To resolve both bugs, separate queries that accounted for the other three

classes of comments were centralized into a new function, has_comments(),

and a query for general comments was introduced here as well.  Furthermore, with 

respect to item 2, the logic for determining the presence of any comments with 

issues is now conditioned on review.ship_it, since this is the only state under 

which this information is necessary.  Previously this was calculated 

unconditionally, which was overkill.
To facilitate consistency and testability, a FIX_IT_THEN_SHIP_IT_TEXT 

constant has been added to the Review class to hold the 

"Fix it, then Ship it!" text.  This is analogous to SHIP_IT_TEXT which

already exists.  These constants are now utilized in the review email templates

rather than the prior string literals.  This introduces one minor delta for

Ship It emails.  Previously, the string literal was "Ship it!", whereas the

string in SHIP_IT_TEXT is "Ship It!'.  That is, the "I" in "It" is now 

capitalized.
Also, added GeneralComment to the list of comment classes discussed in the 

documentation for the CommentDetailDisplayHook.

Augmented all existing unit tests related to review emails with Ship It to

verify that "Fix it, then Ship it!" does not appear in the message.
Added a new unit test for review emails with a Ship It and general

comments.  This is analogous to existing tests for the other three classes of 

comments.  This test would fail without the fix.
Added new unit tests for all four classes of comments to further test review

email's with a Ship It and comments with an opened issue.  Prior to the fix,

the specific test for the general comment class would fail.

Confirmed all unit tests pass.  Built docs and reviewed.