Ensure DB schema uses microsecond precision

[julen]

MySQL versions prior to 5.6.4 didn't support microsecond precision, and whenever microseconds were specified, these were ignored. Versions 5.6.4+ support microsecond precision fine, with an important gotcha though: if the DB schema didn't define microsecond precision (datetime(0), default for <5.6.4), passing microseconds will result in rounding.
I.e. a datetime such as 2016-03-31 23:59:59.999999 would be stored as 2016-04-01 00:00:00.000000

This is a MySQL bug (68760) which can easily make people shoot themselves in the foot, and can (in some edge cases, critically) affect Pootle installations which were created in older versions of MySQL, then upgraded to 5.6.4+, but didn't adjust the underlying DB schema to enable support for microseconds.

We should have some check/data migration which ensures installations using 5.6.4+ have the correct schema definition. Note new installations are not affected by this.