Search English string on entire Project / DistroSeries

This is a follow up of bug 44 comment #43 https://bugs.edge.launchpad.net/rosetta/+bug/44/comments/43

2. Provide searching through English strings in entire project (eg. Ubuntu, to find which POFile a certain string is in, to help with finding odd untranslated string)

As a start we can search only "original" string, as search results can be shared/cached for all languages.

This will solve the use case when someone is searching for an untranslated string

Search results can be batched, to only show the first 10 templates containing that string.

On the results page, as a start, we can only indicate the fact a template contains that string.
The link for that template will do a further search specific to that template, showing all results.

Later we can investigate how to search for a translated string across all templates from a product/distroserie.