[2.x] Improve the media file instance API

[caendesilva]

Abstract

Makes large improvements to the Asset API #1904 by creating a more fluent interface for interacting with media assets. It also removes the media link helper (replacing #1914), now using the Asset API for all media link retrievals. This has the side effect of media files now being required to exist in the project, and that cache busting keys are now consistently added to all media links, and media links will now be qualified using the base URL if set.

Auxiliary Changes

The filesystem facade has been updated to contain the mime type finder.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.97%. Comparing base (620c972) to head (ff18348).
Report is 71 commits behind head on normalize-the-asset-api.

Additional details and impacted files

@@                      Coverage Diff                      @@
##             normalize-the-asset-api    #1932      +/-   ##
=============================================================
- Coverage                     100.00%   99.97%   -0.03%     
- Complexity                      1886     1887       +1     
=============================================================
  Files                            193      193              
  Lines                           4978     4983       +5     
=============================================================
+ Hits                            4978     4982       +4     
- Misses                             0        1       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[caendesilva]

I started adding back support for media files not backed by a file on disk c48a731, but realized that won't make sense, because how would they be used? If they have no contents we have nothing to copy to the output directory.