[v1.3.2] Add Undo and Redo Commands

[esmanda3w]

No description provided.

[codecov-io]

Codecov Report

Merging #157 into master will decrease coverage by 0.47%.
The diff coverage is 56.38%.

@@             Coverage Diff              @@
##             master     #157      +/-   ##
============================================
- Coverage     68.35%   67.87%   -0.48%     
- Complexity      670      686      +16     
============================================
  Files           117      120       +3     
  Lines          2335     2428      +93     
  Branches        328      342      +14     
============================================
+ Hits           1596     1648      +52     
- Misses          617      647      +30     
- Partials        122      133      +11     

Impacted Files	Coverage Δ	Complexity Δ
...n/java/seedu/pivot/logic/commands/RedoCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...n/java/seedu/pivot/logic/commands/UndoCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...ain/java/seedu/pivot/logic/parser/PivotParser.java	76.19% <0.00%> (-8.03%)	12.00 <0.00> (ø)
src/main/java/seedu/pivot/model/Model.java	50.00% <ø> (ø)	1.00 <0.00> (ø)
src/main/java/seedu/pivot/model/ModelManager.java	85.71% <40.00%> (-14.29%)	24.00 <1.00> (+1.00)	⬇️
...rc/main/java/seedu/pivot/model/VersionedPivot.java	70.21% <70.21%> (ø)	15.00 <15.00> (?)
.../java/seedu/pivot/logic/commands/ClearCommand.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
...n/java/seedu/pivot/logic/commands/EditCommand.java	94.04% <100.00%> (+0.07%)	12.00 <0.00> (ø)
...ot/logic/commands/casecommands/AddCaseCommand.java	88.88% <100.00%> (+0.65%)	8.00 <0.00> (ø)
...c/commands/casecommands/AddDescriptionCommand.java	89.28% <100.00%> (+0.39%)	9.00 <0.00> (ø)
... and 12 more

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[CodyChew]

LGTM.