Extract isUndo, isRedo into UndoRedo model

[chia-yh]

Summary:

Extract isUndo and isRedo into the UndoRedo model.

Originally part of the changes in #1197 . However, as #1197 is not being merged, these changes were extracted to this PR as they were viewed as good to have.

Changes Made:

• isUndo, isRedo extracted into UndoRedo

Proposed Commit Message:

Extract isUndo, isRedo into UndoRedo model

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.04% ⚠️

Comparison is base (1b6585f) 53.97% compared to head (2091b50) 53.93%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1205      +/-   ##
==========================================
- Coverage   53.97%   53.93%   -0.04%     
==========================================
  Files         101      101              
  Lines        2881     2881              
  Branches      534      534              
==========================================
- Hits         1555     1554       -1     
  Misses        984      984              
- Partials      342      343       +1     

Files Changed	Coverage Δ
src/app/core/models/undoredo.model.ts	69.64% <0.00%> (-8.36%)	⬇️
.../shared/comment-editor/comment-editor.component.ts	16.84% <0.00%> (+0.53%)	⬆️

... and 1 file with indirect coverage changes

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[gycgabriel]

LGTM