Improvements of indexers

- implementation of batch commits when multiple documents are indexed at
once
- methods for removing a specific document and all documents from the
index were added
- other minor fixes
LinkedIn Manager upgrade

- It is now possible to delete a LinkedIn post
- to get posts with more details from the EEG portal group, one must
specify their number and the starting post index (modified due to the
LinkedIn API limitation of max. 50 returned posts)
Created the indexing service for DB and LinkedIn indexing
Fulltext search classes added

They don't provide any functionality yet.
New fulltext-search-related tests added

- tests on indexing and deleting LinkedIn posts
- IndexingService class test (indexing the database and LinkedIn posts)