Allow specifying "robots" meta tag for pages

Plan:

1. add MetaRobotsExclusions field to "Categories" table (update "install_schema.sql" and "upgrades.sql" files as well)
2. add MetaRobotsExclusions field to "c" and "st" units config files (see field declaration in associated discussion)
3. show MetaRobotsExclusions field during category adding/editing (allow selecting multiple values)
4. add new type value for st_PageInfo tag called "meta_robots", that would return comma-separated list of selected values for given page
5. make changes in "html_head.elm.tpl" of "advanced" theme to demonstrate usage of new type value of st_PageInfo tag
6. add needed phrases to the language pack