Do not show Build button when user is not project admin

[humitos]

Closes #3303

[safwanrahman]

I believe staff user should have permission to trigger any build. Sometime its necessary to trigger build for other projects for debugging purpose. But current pull request will stop doing it.

[humitos]

Well, if we want to change the permissions we also will have to change the view that I wrote the other day: Anthony told me to use the for_admin_users query at https://github.com/rtfd/readthedocs.org/blob/f93e509f1a1c33fe8040b4db06735fdb1a4876c3/readthedocs/builds/views.py#L43-L46

Otherwise, without the current PR, #3303 happens.

[agjohnson]

We can tweak this later. For now it should match the logic that we're already checking against in the API endpoint.

[agjohnson]

I'll open up another issue that will be low priority. It would be nice to have "build latest project version" and "build version" admin functions. I find myself wanting these very often.