WIKI-846: [Advanced Search] Wiki Page's content inside Search Result

Problem analysis:
* WIKI-748 changes the indexing configuration for Wiki Page's content.
  The indexation and then searching is done inside Wiki Page's content instead of the whole Wiki Page node.
  When the keyword is found in Wiki Page's content, the result is therefore a wiki:attachment.

Fix description:
* Handle correctly the excerpt when the result is a Wiki Page's content.
Merge commit '664a9afe7096cdd3e0a0dd269ca715af96dcf206' into HEAD