mysql: change collation of tag table

mysql: change collation of tag table

utf8mb4_unicode_ci considers that 'caché' is equal to 'cache' which
can lead to attaching incorrect tags to entries. This issue is due to
some unicode normalization done by MySQL.

utf8mb4_bin makes no unicode normalization, letting wallabag to consider
'cache' and 'caché' as two different tags.

We change the collation of the whole table as Doctrine does not support
setting a collation on a column for a specific platform (it tries to
apply utf8mb4_bin even for pgsql and sqlite).

Fixes #3302

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>