チャンネルページでisArchivedを明確に表示するように

[Npepperlinux]

• チャンネルがアーカイブされている場合に、「このチャンネルはアーカイブされています」と表示されるようにしました。また、ノートボタンが表示されなくなりました（Thanks @4ster1sk !）
  

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 40.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 27.61%. Comparing base (cb8696b) to head (3ed72b4).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
lib/view/channels_page/channel_detail_page.dart	25.00%	6 Missing ⚠️
lib/view/channels_page/channel_detail_info.dart	57.14%	3 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #671      +/-   ##
===========================================
+ Coverage    27.59%   27.61%   +0.01%     
===========================================
  Files          312      312              
  Lines        16454    16461       +7     
===========================================
+ Hits          4541     4545       +4     
- Misses       11913    11916       +3     

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[4ster1sk]

書き込みボタン非表示はChannelDetailFloatingActionButtonのAsyncData(:final value)でComminityChannelとれるのでそのへんでいい感じにすれば良さそう