Remove tag feature

[cshao02]

The existing tag functionality is irrelevant for the target group of student event planners that we chose.

As such , it is wise to remove unnecessary features such that the product can function more cohesively with a set of features that are more suited to the target users.

This PR, when merged, will result in the removal of the existing tag fuctionality and change telegram username field to use (t/) instead of (T/) which is more efficient. The tags will also disappear from GUI.

closes #74

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/main/java/seedu/address/logic/Messages.java	80.00%	0 Missing and 1 partial :warning:
...java/seedu/address/logic/commands/EditCommand.java	66.66%	0 Missing and 1 partial :warning:

Files with missing lines	Coverage Δ	Complexity Δ
.../java/seedu/address/logic/commands/AddCommand.java	100.00% <ø> (ø)	8.00 <0.00> (ø)
...a/seedu/address/logic/parser/AddCommandParser.java	100.00% <100.00%> (ø)	5.00 <0.00> (ø)
...ain/java/seedu/address/logic/parser/CliSyntax.java	85.71% <100.00%> (-1.79%)	1.00 <0.00> (ø)
.../seedu/address/logic/parser/EditCommandParser.java	100.00% <100.00%> (+6.66%)	8.00 <0.00> (-4.00)	:arrow_up:
...in/java/seedu/address/logic/parser/ParserUtil.java	87.17% <ø> (-2.83%)	14.00 <0.00> (-4.00)
...c/main/java/seedu/address/model/person/Person.java	92.85% <100.00%> (-0.76%)	27.00 <1.00> (-2.00)
...a/seedu/address/model/person/TelegramUsername.java	83.33% <ø> (+5.55%)	9.00 <0.00> (+1.00)
.../java/seedu/address/model/util/SampleDataUtil.java	0.00% <ø> (-20.00%)	0.00 <0.00> (-1.00)
.../java/seedu/address/storage/JsonAdaptedPerson.java	100.00% <100.00%> (ø)	11.00 <0.00> (ø)
src/main/java/seedu/address/ui/PersonCard.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
... and 2 more