Migrate pallet-offences to pallet attribute macro

[stanly-johnson]

Part of #7882.

Converts the Offences pallet to the new pallet attribute macro introduced in #6877.

Following the upgrade guidelines here: https://crates.parity.io/frame_support/attr.pallet.html#upgrade-guidelines.

⚠️ Breaking Change ⚠️

From https://crates.parity.io/frame_support/attr.pallet.html#checking-upgrade-guidelines

storages now use PalletInfo for module_prefix instead of the one given to decl_storage: Thus any use of this pallet in construct_runtime! should be careful to update name in order not to break storage or to upgrade storage (moreover for instantiable pallet). If pallet is published, make sure to warn about this breaking change.

[cla-bot-2021]

User @stanly-johnson, please sign the CLA here.

[shawntabrizi]

some other spots where tabs needed too

[shawntabrizi]

Also compile locally to check for unused imports like this:

error: unused import: `StorageMap`
  --> frame/offences/src/mock.rs:34:19
   |
34 |     parameter_types, StorageMap, StorageDoubleMap,
   |                      ^^^^^^^^^^
   |
   = note: `-D unused-imports` implied by `-D warnings`
error: unused import: `StorageDoubleMap`
  --> frame/offences/src/mock.rs:34:31
   |
34 |     parameter_types, StorageMap, StorageDoubleMap,

[gui1117]

looks good to me

[shawntabrizi]

bot merge

[ghost]

Waiting for commit status.