Remove the (undocumented) `Database.MysqlCharset` setting, and
introduce `Database.DefaultCharset` and `Database.DefaultCollation`
settings instead.

The reason for the `MysqlCharset` removal is that in a later patch,
we're going to adjust the engine initialization so that it achieves
similar results, in a different way.

The new settings will remain undocumented for similar reasons
`MysqlCharset` was undocumented: the defaults work for the vast majority
of cases, and it's too easy to break things if changing them.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

Replace `db.ConvertUtf8ToUtf8mb4()` with
`db.ConvertCharsetAndCollation()`, which does essentially the same
thing, but the charset and the collation is controlled by the caller.

Also introduce `db.findCaseSensitiveCollation()` which attempts to
auto-detect the best supported case-sensitive collation for MySQL and
MariaDB databases.

With these two functions, `gitea doctor check` can be adapted to use the
charset and collation set in the configuration (or auto-detect them, if
unset), and this patch does that too.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

When starting the web server, perform a sanity check on the database.
The sanity check currently runs for MySQL/MariaDB only, and verifies
that the charset and collation are correctly set on both the database,
and on all tables in it.

If there is a discrepancy, it does not error out, but prints a warning
only.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

When initializing an empty MySQL/MariaDB database, ensure that the
character set and collation is set to the desired values.

With these set, creating a table will inherit these settings, unless
table creation specifies a different charset or collate function. This
is the reason why `setting.Database.MysqlCharset` was removed: that
forced table creation to explicitly set a charset, rather than inherit
the database's default, and in doing so, also changed the collate
function to the charset's default (which may - and usually is -
different from the one we want).

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

For MySQL, suggest not setting a collate function, to let Gitea deal
with it. But also mention some of the options, would one want to set it
up in advance and not rely on Gitea.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>