Fix mimetype -> content_type
[bug 1093341] Tweak Gengo account balance warning

This reduces the number of times we get balance warning emails--we
should only get them once a translation sync.

We should further reduce this by having the code only run once a day,
but I'm going to push that off to a later pass.
Merge pull request #375 from willkg/1093341-gengo-warning

1093341 gengo warning
[bug 1094197] Fix JSONObjectField default default

This fixes the default so that it's always u'{}'. It can be overridden,
so mostly this just codifies the default so we don't have to keep typing
it across the codebase.
Merge pull request #376 from willkg/1094197-fix-jsonobjectfield

[bug 1094197] Fix JSONObjectField default default