GitLab

Replaces .card-header > hX.card-title with hX.card-header to remove
.card-title. At a occurrence where bold styling is not necessary,
div.card-header is used instead.
By full removal of .card-title at the moment, all workarounds for
BS4's card-title are removed as well.

More information: https://getbootstrap.com/docs/4.0/migration/#panels

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>