(bug 47820) Change $wgSitename and NS_PROJECT_TALK for udmwiki

This commit does the following things:
* (1) It renames $wgSitename for udmwiki from the default 'Wikipedia'
into a localised version in Udmurt;
* (2) It renames NS_PROJECT_TALK from the current half-translated
version into a correct one, fully translated;
* (3) It provides an alias for the current ('old') name of this
namespace not to break existing links.

Even though the bug mentions other namespaces (NS_FILE and
NS_FILE_TALK), there is no need to update them here, as translations
are managed on Translatewiki.

Bug: 47820
Change-Id: Id5b0b82839392dfd9ae51119e2ab2d51b13938e3