Rename 'Sort by' with 'Filter by' or display also entries with 0 count

The current forum doesn't have a sort functionality, instead it provides a filter functionality.
The difference is that:

• the sort should preserve the entries while changing the order;
• filter displays only the entries matching a specific criteria.

The confusion can be easily fixed by changing the translation from 'sort' to 'filter', on:

• topics view;
• 'Most Active Topics' panel;