Reinstate keyword-filter identifier.

Removed in a previous cleanup the containing element for the keywords
field on all document filtering needs to have a filter specific
elementId.
See:
https://github.com/alphagov/whitehall/commit/4c682d099fa0f77f042d01bcd39c5dd9d44f317d#diff-08e4a107c3401662ebf39b86de38b01cL11
Merge pull request #1777 from alphagov/fix_document_filter_text_search

Reinstate keyword-filter identifier.