Add Mastodon link to server email templates

[jc0b]

• Adds a link to FleetDM's Mastodon account to emails sent by the FleetDM server
• Adds a Mastodon SVG image to the repo

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.35%. Comparing base (6b62f3f) to head (92298ed).
Report is 64 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #23309    +/-   ##
========================================
  Coverage   63.35%   63.35%            
========================================
  Files        1538     1545     +7     
  Lines      144703   144846   +143     
  Branches     3601     3675    +74     
========================================
+ Hits        91670    91771   +101     
- Misses      45798    45839    +41     
- Partials     7235     7236     +1     

Flag	Coverage Δ
frontend	52.42% <ø> (+0.23%)	⬆️

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[rachaelshaw]

looks like other logos are .png and named <something>-logo-<dimensions>@2x.png — we should follow those conventions

[jc0b]

Ah, I just took the SVG from Mastodon's brand toolkit. Because these are vector graphics it didn't really feel appropriate to add a resolution (as it technically doesn't have one). I can convert it to PNG of the same size if that's more desirable.

[jc0b]

I just checked and there are a few other SVG graphics in that same directory. Let me know if you want the Mastodon one converted to a 48x48 PNG :)