Make page content blocks searcheable

1. create "PageHelper::getRevisionContent($page_revision_id)" method, that will: - 0.5h
   1. get contents of all content blocks (they belong to single page) in a given revision
   2. process content from each language separately
   3. combine content block content using space and ignoring empty values
   4. call the "html_entity_decode" function with same arguments as "htmlspecialchars_decode" in "kUtil::unescape" (replaces " " and similar things with searchable equivalents)
   5. call the "strip_tags" (removed HTML tags)
   6. return associative array, where each key would be in "lX_PageContent" format (X is language ID) and value would be searchable revision content on that language
2. add "lX_PageContent" column (X is language id) to the "PageRevisions" table: - 0.5h
   1. add multi-lingual field declaration in the unit config
   2. create "CoreUpgrades::Upgrade_5_2_2_B3" method (see similar methods in that class), that will only when "$mode == 'after'":
      1. call "\kMultiLanguageHelper::createFields" method to create new multilingual fields in "PageRevisions" table
      2. populate created fields using "PageHelper::getRevisionContent" method for each of existing page revisions
3. create "page-revision:OnAfterPageContentChangedHook" event, that will: - 0.5h
   1. be a hook to "content:OnAfterItemCreate", "content:OnAfterItemUpdate" and "content:OnAfterItemDelete" events
   2. load page revision object associated with affected content block
   3. call "PageHelper::getRevisionContent" method to rebuild content
   4. store above retrieved content to loaded page revision in database
4. add record to "SearchConfig" table to: - 0.5h
   1. appear in "Configuration > Website > Search" section
   2. make "PageRevisions.lX_PageContent" (X is language ID) of currently approved page revision (the Revision number (NOT ID) is stored in "Categories.LiveRevisionNumber" column)

Quote: 2h*1.4 = 3h

Test with "Enable Revision Control for Page Content" setting turned and and off so that in case of multiple page content presence only approved version is searchable.