Make use of Perbill in staking/phragmen for readibility

[gui1117]

We still want to keep PerU128 as it is, but we can get read of ACCURACY by replacing operation using ACCURACY by a Perbill implementation.

Note that using Perbill implementation is less precise than the current const. If we really want that precision we can create a PerU32 in the same way of Perbill, or even a PerU64 is actually implementable (because we need double space and u128 is available).

This would improve readibility of the code.

follows #2889,

see this comment #2889 (comment)