feat: request subscription to group/channel for download files

[mr-linch]

Task

close #6

• [x] define Chat core
• [x] define Chat store
• [x] define Chat service
• [x] implement connection of chats
• [x] implement update of chats when title changed;
• [x] implement check of subscription
• [x] ~send notification when user subscribe to channel for download;~
• [x] stats in file view;
• [x] stats in chat view;

Housekeeping

• [x] use new way of work with store;
• [x] add integration test for stores;

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@862ff3c). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #35   +/-   ##
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  Coverage          ?   20.39%           
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  Files             ?       10           
  Lines             ?      505           
  Branches          ?        0           
=========================================
  Hits              ?      103           
  Misses            ?      388           
  Partials          ?       14           

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