[WIP] Office mime types update fix

:no_entry: :construction: WIP

To be able to debug different migration situations, I have moved the office mime type code to a repair step.

The second test case will fail for now because the current code doesn't work when the target mime type already exists.

This is a possible fix for https://github.com/owncloud/core/pull/8895

- [ ] add precondition to prevent re-running the same repair steps needlessly
- [ ] fix the "UPDATE" SQL statement for the case where the target value already exists
- [ ] investigate mime type discrepancies on master (see https://github.com/owncloud/core/pull/8895#issuecomment-47134711)

@butonic @icewind1991 
Yes, but not as part of this PR. It does currently provide a neat way to execute a plaing string, a statement or an array with 'sql' and optionally 'limit' and 'offset' keys. The only overhead is checking if the results are error objects of some kind. But you can create a technical debt issue for that!