Drop scap_content#digest

No need for it since #2.
Allow search based on scap_content#title and filename

Recently, the search on scap_content#digest has been dropped. However,
some search needs to be defined to enable scap_content.search_for
method.

Addressing:
Operation FAILED: undefined method `search_for' for
<ActiveRecord::Relation:0x0000000aab0838>