switch to utf8mb4 which can store all unicode code points including
emoticons (utf8 which we used before is an alias for utf8mb3 which only
stores a maximum of three bytes).

the rake task cg:convert_to_unicode converts the database into utf8mb4.
it also sets a binary collation for our tags table, because we
want to distinguish between olé and ole. there is no test for this
feature (because it would fail).
the schema_migrations table has to be excluded, because it has
indexes which are to long

the emoji tests work without database conversion, because database
connection uses utf8mb4.