storage: Improve index on content_ctags

Work on the suggestion message from postgresql

psycopg2.OperationalError: index row size 3992 exceeds maximum 2712 for
index "content_ctags_id_name_kind_line_lang_idx" HINT: Values larger
than 1/3 of a buffer page cannot be indexed.  Consider a function index
of an MD5 hash of the value, or use full text indexing.

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