Combine years in the chart footer when the same data providers appears multiple times, e.g. "World Bank (2011; 2012)"

[pabloarosado]

Core problem

When we have two origins with the exact same producer, but different years, what we show in the chart footer is repetitive, e.g. "World Bank (2011); World Bank (2012)".

Proposed solution

We could possibly combine them, e.g. "World Bank (2011; 2012)" or "World Bank (2011, 2012)" (since ";" is used to separate different origins).

Context

This issue came up in https://github.com/owid/owid-issues/issues/1630