Make link immutable

[xiangjunn]

• Make link and unlink immutable
• Shows error message if contact already linked/not linked to event
• Fix cedit and eedit allowing the removal of a non-existing tag
• Make link and unlink work with undo
• Fix cedit and eedit allowing the adding of a already existing tag

[codecov]

Codecov Report

Merging #218 (bd4dc5b) into master (1e72508) will increase coverage by 0.47%.
The diff coverage is 89.93%.

@@             Coverage Diff              @@
##             master     #218      +/-   ##
============================================
+ Coverage     68.94%   69.41%   +0.47%     
- Complexity     1004     1029      +25     
============================================
  Files           129      129              
  Lines          3355     3404      +49     
  Branches        433      450      +17     
============================================
+ Hits           2313     2363      +50     
+ Misses          857      849       -8     
- Partials        185      192       +7     

Impacted Files	Coverage Δ
...rc/main/java/seedu/address/logic/LogicManager.java	51.51% <0.00%> (-1.43%)	⬇️
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
.../seedu/address/storage/JsonAddressBookStorage.java	100.00% <ø> (ø)
...u/address/storage/JsonSerializableAddressBook.java	84.61% <ø> (-0.57%)	⬇️
src/main/java/seedu/address/model/event/Event.java	79.59% <78.94%> (-2.13%)	⬇️
...edu/address/logic/commands/event/ELinkCommand.java	89.47% <87.50%> (-3.39%)	⬇️
...u/address/logic/commands/event/EUnlinkCommand.java	89.13% <87.50%> (-2.71%)	⬇️
...rc/main/java/seedu/address/model/ModelManager.java	79.51% <90.90%> (-1.13%)	⬇️
src/main/java/seedu/address/model/AddressBook.java	93.25% <91.66%> (+4.79%)	⬆️
...u/address/logic/commands/contact/CEditCommand.java	94.87% <92.85%> (-0.37%)	⬇️
... and 19 more

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[chunweii]

Some minor but important changes. Otherwise, looks good.

[chunweii]

Also, please add more test cases. See codecov report to see where you are lacking.

[chunweii]

LGTM, but there are some other changes to be made

[chunweii]

When the user tries to delete a tag that does not exist, the program will give an error and show a red message. However, if the user tries to unlink something that is not linked, the program allows it but displays the message that it is not linked. This is quite inconsistent. Also, you forgot to implement the check for adding new tags if there are already duplicate tags in CEdit and EEdit commands.

[chunweii]

Since I made changes to the PR, I shall not approve this PR. Even though it is showing green, please wait for Janice review

[janjanchen]

LGTM :) Good job!!