[WIP] Remove the ability to add identities as MS signers

[adamdossa]

changelog

modified API

• extrinsics create_or_approve_proposal_as_identity, create_or_approve_proposal_as_key will now always fail
• extrinsics create_proposal_as_identity, approve_as_identity, reject_as_identity, accept_multisig_signer_as_identity will now always fail
• adding an Identity as a signer on a multisig will now always fail

modified logic

• No longer permit Identity based multisig signers
• No longer permit create_or_approve approach to multisig transactions

[codecov[bot]]

Codecov Report

Merging #1555 (f06ccdb) into develop (9734963) will decrease coverage by 0.46%. The diff coverage is 88.13%.

@@             Coverage Diff             @@
##           develop    #1555      +/-   ##
===========================================
- Coverage    58.58%   58.12%   -0.46%     
===========================================
  Files          142      142              
  Lines        32104    32116      +12     
===========================================
- Hits         18807    18669     -138     
- Misses       13297    13447     +150     

Files	Coverage Δ
pallets/runtime/common/src/fee_details.rs	58.75% <0.00%> (+0.72%)	:arrow_up:
pallets/multisig/src/lib.rs	71.17% <89.65%> (-14.13%)	:arrow_down:

... and 5 files with indirect coverage changes