DOC-3043 Wiki JAVA APIs: Improve English and consistence
WIKI-746 Slow invocation of PageImpl.checkin for wiki page update

Fix description:
* When a wiki page is modified, a new version will be created only for wiki's content child node instead of the whole wiki page node.
  That allows to improve much the performance.
* Migrate legacy wiki page on the fly, when the user opens the wiki page itself.
WIKI-681 Improve the loading speed of search scope at Wiki Advanced Search Form

Fix description:
* Use Space Switcher to choose search scope in Wiki Advanced Search.
  There is no longer all users' wiki and therefore improves performance
Merge commit 'a1e9facbcdf6e794a0a280e4e5da5dbbb0767c2a' into HEAD