Make "the authors" translatable

[ErinBecker]

Fixes #579 by removing the hard coding English for {authors} and instead making the authors part of the {MaterialsLicensedUnder} variable. Also updates translation string for R-es.po.

Screenshot of Spanish language footer before:

Screenshot of Spanish language footer after:

[joelnitta]

@ErinBecker please see ErinBecker#1 with JA translation

[ErinBecker]

The build check for Windows 4.1 is failing with the following output:

  Running 'testthat.R' [239s]
 ERROR
Running the tests in 'tests/testthat.R' failed.
Last 13 lines of output:
  -- Failure ('test-move_episode.R:15:3'): all episodes are present in the config file --
  `eps` (`actual`) and fs::path_file(get_sources(res, "episodes")) (`expected`) don't have the same values.
  * Only in `actual`: "data", "fig", "files"
  
  
  [ FAIL 9 | WARN 2 | SKIP 66 | PASS 929 ]
  Error: Test failures
  i removing 'sandpaper-local' ('D:/a/_temp/Rtmp0wmdmM/working_dir/RtmpO68jjj/REMOTE-15cc59a5ed2')
  x Error trying to remove remote
  [EPERM] Failed to remove
  'D:/a/_temp/Rtmp0wmdmM/working_dir/RtmpO68jjj/REMOTE-15cc59a5ed2/objects/pack/pack-55abd0f3f2d77afd2575a4b80c4e649ef9696955.idx':
  operation not permitted
  i 'D:/a/_temp/Rtmp0wmdmM/working_dir/RtmpO68jjj/file15cc54f25a92/lesson-example' successfully removed
  i local remote could not be removed
  Execution halted
* checking for unstated dependencies in vignettes ... OK
* checking package vignettes in 'inst/doc' ... OK
* checking re-building of vignette outputs ... OK
* checking for non-standard things in the check directory ... OK
* checking for detritus in the temp directory ... OK
* DONE
Status: 1 ERROR, 3 NOTEs

There are also failed checks starting at line 625

Need to do more testing to understand these errors.

[froggleston]

These errors were spurious and specific to that build. Rerunning seemed to work, so merged!