plug gravatar field on api

                            
|Q            |A           |
|---          |---         |
|Bug Fix?     |no          |
|New Feature? |yes         |
|BC Breaks?   |no          |
|Deprecations?|no          |
|Tests Pass?  |yes         |
|Fixed Tickets|#JOINDIN-464|
|License      |MIT         |
|Doc PR       |            |
                            


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Todo:

- [x] plug field on user resource 
- [x] plug field on the comments talk and even resources
- [x] ~~add fixtures to test these puppies~~
not going to do the latter because it is sql and i think this just works or will need to be tested anyways by hand so i will let you ponder this PR and merge it whenever you deem it possible.