Replace Twitter with Mastodon in footer

[mvexel]

Fixes #2261.

• Remove reference to twitter account in readme
• Replace twitter with mastodon in footer
• Remove twitter opengraph meta links

Removing the OpenGraph links is not strictly necessary, but I don't care for MapRoulette being twitter-friendly at all. Open for discussion though.

[mvexel]

Sorry about the re-formatting. I turned of format on save for this project.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ca1f85d) 23.64% compared to head (462c527) 23.64%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2265   +/-   ##
=======================================
  Coverage   23.64%   23.64%           
=======================================
  Files         648      648           
  Lines       22410    22410           
  Branches     6921     6921           
=======================================
  Hits         5298     5298           
  Misses      14303    14303           
  Partials     2809     2809           

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