Attempt to fix flaky AMO tests by not relying on ExtraAppTestCase anymore

ExtraAppTestCase causes problems depending on import order, because
any models with FKs that is imported get cached into its related
model's _meta._related_objects_cache.
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Merge branch 'master' of ssh://github.com/mozilla/zamboni
Merge branch 'master' of ssh://github.com/mozilla/zamboni
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Commit from Mozilla Verbatim by user smolejv@gmx.net.: 1937 of 1937 strings translated (0 fuzzy).
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set tabzilla to default z-index (bug 927168)
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Commit from Mozilla Verbatim by user marsf.: 1796 of 5452 strings translated (27 fuzzy).
Commit from Mozilla Verbatim by user marsf.: 1797 of 5452 strings translated (27 fuzzy).
Commit from Mozilla Verbatim by user shaohua.wen@gmail.com.: 4597 of 5088 strings translated (0 fuzzy).
a minor, but key, fix
fixed search merorizing version (bug 729414)
Merge pull request #1267 from diox/fix-flaky-tests

Attempt to fix flaky AMO tests by not relying on ExtraAppTestCase anymore
Merge pull request #1264 from vinyll/version_sidebar-729414

fixed search merorizing version (bug 729414)
Commit from Mozilla Verbatim by user kelemeng.: 2169 of 5452 strings translated (3 fuzzy).
Commit from Mozilla Verbatim by user toniher@softcatala.org.: 2080 of 5452 strings translated (7 fuzzy).
Commit from Mozilla Verbatim by user michael.koehler1@gmx.de.: 5452 of 5452 strings translated (0 fuzzy).