テーマをインストールできるように

[poppingmoon]

Resolve #383

テーマのインストールと削除ができるようにしました

• shared preferencesにテーマコードを保存します
• 組み込みテーマと同様にidで指定します

インストールダイアログ	インストールされたテーマ一覧	テーマの選択	テーマの適用後

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 21.05263% with 105 lines in your changes missing coverage. Please review.

Project coverage is 27.67%. Comparing base (8512cea) to head (69d0004).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...e/general_settings_page/installed_themes_page.dart	2.08%	47 Missing ⚠️
...ge/general_settings_page/install_theme_dialog.dart	2.85%	34 Missing ⚠️
...lled_themes_page/misskey_theme_codes_notifier.dart	45.45%	18 Missing ⚠️
...e/general_settings_page/general_settings_page.dart	0.00%	6 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #386      +/-   ##
===========================================
- Coverage    27.73%   27.67%   -0.07%     
===========================================
  Files          312      316       +4     
  Lines        16296    16427     +131     
===========================================
+ Hits          4520     4546      +26     
- Misses       11776    11881     +105     

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