Currently translated at 100.0% (792 of 792)

- sends notification as merge failure
- logs and error message
- fixes issue #224

The code was duplicated in two places, having just one in unit manager
is saner.

Otherwise we always see our change record

- going beyond last unit is properly detected
- search result is freed when leaving search
- fixed link to last unit

Otherwise results might stay in session forever.

It is not really that useful to be loaded for every string to translate.

Now also properly labels review.

The params should be processed in form, not further in the code.

Search is no longer performed on each request, but rather once and then
results are cached.

- correct message for no search results (issue #219)
- allows to go back in translated strings (issue #229)