Refactor Authors, Sources and Tags classes

[H4mes]

Added classes for author, source and tag.

Added tag pill UI elements for authors, sources and tags.

[codecov]

Codecov Report

Attention: Patch coverage is 0.53191% with 187 lines in your changes are missing coverage. Please review.

Project coverage is 52.64%. Comparing base (585ed77) to head (8246758).
Report is 2 commits behind head on master.

Files	Patch %	Lines
...java/seedu/address/storage/JsonAdaptedArticle.java	0.00%	34 Missing ⚠️
...main/java/seedu/address/model/article/Article.java	0.00%	32 Missing ⚠️
...c/commands/articlecommands/EditArticleCommand.java	0.00%	15 Missing ⚠️
...address/logic/parser/EditArticleCommandParser.java	0.00%	14 Missing ⚠️
.../main/java/seedu/address/model/article/Author.java	0.00%	14 Missing ⚠️
.../main/java/seedu/address/model/article/Source.java	0.00%	14 Missing ⚠️
...in/java/seedu/address/logic/parser/ParserUtil.java	0.00%	12 Missing ⚠️
...eedu/address/model/util/SampleArticleDataUtil.java	0.00%	11 Missing ⚠️
.../java/seedu/address/storage/JsonAdaptedAuthor.java	0.00%	10 Missing ⚠️
.../java/seedu/address/storage/JsonAdaptedSource.java	0.00%	10 Missing ⚠️
... and 3 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #60      +/-   ##
============================================
- Coverage     55.38%   52.64%   -2.75%     
  Complexity      451      451              
============================================
  Files            97      101       +4     
  Lines          1959     2061     +102     
  Branches        200      216      +16     
============================================
  Hits           1085     1085              
- Misses          835      937     +102     
  Partials         39       39              

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[Ko-Khan]

LGTM, but maybe check the tests to ensure they are still working, as you changed a lot of old code

[bennyLCK]

Some possible flaws to rectify are present.

[bennyLCK]

LGTM