In `admin/database-preparation` suggest `utf8mb4_bin` as the collate
function, rather than `utf8mb4_unicode_ci`. The former is accent- and
case sensitive, while the latter isn't, and Forgejo assumes that columns
are case sensitive.
Also add a short paragraph explaining why `utf8mb4_bin` is
suggested (case sensitivity), and what problems may arise and why if
case insensitive collation is used.
This partially addresses forgejo/forgejo#2039.
Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>