Move /site to go.mod and Go 1.12/1.13

[markmandel]

https://github.com/googleforgames/agones/tree/master/site

We do some gopath shenanigans to create and deploy the site:
https://github.com/googleforgames/agones/blob/master/build/includes/website.mk#L50
https://github.com/googleforgames/agones/blob/master/site/cloudbuild.yaml#L42

Which we don't need to do anymore (and bit me the other day during release).

We could move this to a go.mod and also update the App Engine version to a later version of go (currently 1.11) with minimal difficulty, and it would make this process much easier.

[markmandel]

@heartrobotninja still keen on taking this on?

[heartrobotninja]

Yep! I can start on that in a little bit.

…

On Fri, Jan 24, 2020 at 3:59 PM Mark Mandel ***@***.***> wrote: @heartrobotninja <https://github.com/heartrobotninja> still keen on taking this on? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#1295?email_source=notifications&email_token=ABM5DZ2VTCYSQTUKYHJIICDQ7N6ETA5CNFSM4KLNUHB2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEJ4OGNY#issuecomment-578347831>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABM5DZZAHTYREUAFCG55XDLQ7N6ETANCNFSM4KLNUHBQ> .

[markmandel]

Thanks!

/assign @heartrobotninja