Second phase of ID store implementation

[covert-encryption]

Continuation of the work in #81.

TODO:

• [x] Public ID key storage and automatic generation
• [x] Use ID-based keys, display IDs in authentication and signatures instead of raw keys (CLI)
• [x] ID management (key export, changing of passphrase etc) covert id subcommand
• [ ] ID selection dropdowns and other functionality based on ID store (GUI)
• [ ] Daemon (agent) that remembers the passphrase for a while
• [x] Time-based expiration of keys (locally, not planned to be included in messages)
• [x] Code cleanup / refactoring & UX polish
• [ ] Workflow to "reply" to messages in an easy way (CLI, GUI)
• [x] Forward secrecy in conversations

[codecov[bot]]

Codecov Report

Merging #89 (8838299) into main (7a712f5) will increase coverage by 0.11%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   78.92%   79.04%   +0.11%     
==========================================
  Files          34       34              
  Lines        2700     2715      +15     
  Branches      611      616       +5     
==========================================
+ Hits         2131     2146      +15     
  Misses        432      432              
  Partials      137      137              

Impacted Files	Coverage Δ
covert/idstore.py	90.97% <100.00%> (+1.04%)	:arrow_up:
covert/ratchet.py	100.00% <100.00%> (ø)

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