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