Find collection by slug instead of id

[php-coder]

The following methods should be modified to do a search by slug instead of id field:

• categoryService.countCategoriesOf(collectionId)
• categoryService.getStatisticsOf(collectionId, lang)
• countryService.countCountriesOf(collectionId)
• countryService.getStatisticsOf(collectionId, lang)
• seriesService.countSeriesOf(collectionId)
• seriesService.countStampsOf(collectionId)

Tech debt for: b2f1c61 (#207)

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Important note: I expect to have 6 commits (one per each method). It makes the process of review easier and changes more granular. The changes from one modification should belong strictly to a single commit, in other words they should be atomic.