Reciprocal language fallback for Upper/Lower Sorbian (hsb, dsb)

Reciprocal language fallback for Upper/Lower Sorbian (hsb, dsb)

The two are closely related and are estimated to be 86 % mutually
intelligible in written form.

https://translatewiki.net/wiki/Thread:Portal_talk:Hsb/Fallback_to_dsb
https://translatewiki.net/wiki/Thread:Portal_talk:Dsb/Fallback_to_hsb

The two locales already have identical $dateFormats and largely
overlapping $namespaceGenderAliases and translations of namespaces,
magic words and special pages (often they differ only for diacritics
and only about a dozen of them are defined in one but not the other).

All the users with some edits on dsb.wiki, hsb.wiki, hsb.wiktionary
(as listed by WikiStats) have been notified individually.

Change-Id: Ib806387c875c1bbef180f2b85eae45c050824cb7