Add Matrix to list of instant messaging networks

[jtrees]

Fix #1675

[codecov]

Codecov Report

Merging #1761 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1761   +/-   ##
=========================================
  Coverage     27.23%   27.23%           
  Complexity      148      148           
=========================================
  Files            19       19           
  Lines           448      448           
=========================================
  Hits            122      122           
  Misses          326      326           

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[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/contacts/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22
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