Avoid thousands of SQL queries when opening many endpoints in the browser

When opening an endpoint in the browser to look at its queries from the debug toolbar, it is common to wait for quite a while because the HTML form loads up to a 1000 items for all PrimaryKeyRelatedField (#634 (closed), #642 (closed)) so I took a few minutes to just add the usual one-line fix to every single affected PrimaryKeyRelatedField…

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requested review from @babadie

added 6 commits

• c25d89e5...17b2335d - 5 commits from branch master
• f9b78547 - Avoid thousands of SQL queries when opening many endpoints in the browser

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mentioned in commit f95d5e3f