Fix incorrect saving during SortCommand

[rahula1008]

During the SortCommand, if a FindCommand was executed first, which reduces the size of the sortedPersons list, then during the save, contacts not matching the find query get deleted.

To fix this:

• Updated the ModelManager to sort the sortedPersons and sort every contact
• Then, the ModelManager only saves the sortedAllPersons instead of the sortedPersons.

Closes #148

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/main/java/seedu/address/model/ModelManager.java	71.42%	0 Missing and 2 partials :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...java/seedu/address/logic/commands/SortCommand.java	92.59% <ø> (ø)	14.00 <0.00> (?)
...rc/main/java/seedu/address/model/ModelManager.java	92.06% <71.42%> (ø)	24.00 <0.00> (?)