Make year/month/week/day/... proper plurals and not individual strings

[vanillalounge]

In ​http://buddypress.trac.wordpress.org/browser/trunk/bp-core/bp-core-functions.php#L594, the duration strings are being called individually with __() gettext functions when they should be called as a group with _n().

The code currently assumes that there is only one plural form (i.e. 'one' and 'many') when many languages have way more complex plural forms. Polish, for instance has an expression for 1, another for 2--4, 22--24, 32--34, 42--44, ... and yet another for 0, 5--21, 25--31, 35--41, ...

_n() handles these properly, for all languages supported by GlotPress.

[SergeyBiryukov]

Introduced in [624] as a fix for #154. Also mentioned in ticket:190:4.

Related: #3056

[SergeyBiryukov]

5015.patch​ uses _n() for proper i18n.

[boonebgorges]

In 7101:

Adds unit tests for bp_core_time_since()

See #5017. See #5015

[boonebgorges]

Thanks for the report and for the patch. Looks good.

[boonebgorges]

In 7102:

Use _n() for plurals in bp_core_time_since()

The previous implementation used a hardcoded distinction between singular and
plurals, which is not flexible enough for many languages.

Fixes #5015

Props SergeyBiryukov