feat: support customized privileges required reputation

[hgaol]

fix #695 .

what this PR did.

• added a custom privilege level in Admin setting. Using 99 as the custom privilege level.
• added default custom privilege when there's no corresponding settings, which makes it compatible when upgrading from ealier version.
• validated the numbers which should be equal or larger than 1 in both frontend and backend.

[hgaol]

+@LinkinStars and @shuashuai to help to review, thx!