Use custom PR template by default

                    
|Q            |A   |
|---          |--- |
|Bug Fix?     |no  |
|New Feature? |Y   |
|BC Breaks?   |no  |
|Deprecations?|no  |
|Tests Pass?  |Y   |
|Fixed Tickets|#266|
|License      |MIT |
                    

Use the custom PR table by default if it is defined in the .gush.yml file
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