Disallow Identities to join MultiSigs as signers

[adamdossa]

changelog

modified API

• accept_multisig_signer_as_identity will now always fail

modified logic

• No longer permit Identity based multisig signers

[codecov]

Codecov Report

Merging #1556 (76dc10d) into develop (af8f1c3) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1556      +/-   ##
===========================================
- Coverage    58.56%   58.55%   -0.02%     
===========================================
  Files          142      142              
  Lines        32118    32120       +2     
===========================================
- Hits         18811    18807       -4     
- Misses       13307    13313       +6     

Files	Coverage Δ
pallets/multisig/src/lib.rs	85.33% <100.00%> (+0.03%)	⬆️

... and 3 files with indirect coverage changes