fix being able to add overlapping leaves

[Ben-Hanan]

Fixed a bug in being able to add leaves with overlapping periods.
e.g addleave l/d:08/10/2020 d:10/10/2020 l/d:09/10/2020
The above command would not have resulted in an error and added the two leaves as different records of leave but it errors now.

Also made the UI automatically update with any change to leave.

[codecov-io]

Codecov Report

Merging #89 into master will decrease coverage by 0.19%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master      #89      +/-   ##
============================================
- Coverage     46.69%   46.49%   -0.20%     
  Complexity      476      476              
============================================
  Files           109      109              
  Lines          2388     2398      +10     
  Branches        280      283       +3     
============================================
  Hits           1115     1115              
- Misses         1220     1230      +10     
  Partials         53       53              

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/eva/commons/util/DateUtil.java	21.73% <0.00%> (-3.27%)	3.00 <0.00> (ø)
...n/java/com/eva/logic/commands/AddLeaveCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
src/main/java/com/eva/model/Model.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
src/main/java/com/eva/model/ModelManager.java	45.29% <0.00%> (-2.45%)	25.00 <0.00> (ø)

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[Nikhilalalalala]

LGTM! Just some minor comments on the javadocs part. Nice observation on this bug :)

[TheSpaceCuber]

LGTM