Update schemas to allow display defaults for transmission lines

[jameshadfield]

Schemas and test builds updated to allow one to define a display default
in a dataset JSON to toggle on/off the transmission lines (if available).
See the corresponding auspice PR nextstrain/auspice#1152.

Additionally, the logic for setting display_defaults within augur export v2 is
improved so that we no longer have to define the valid keys in the source code.
Invalid keys are still identified via validating the JSONs against schemas.

[codecov]

Codecov Report

Merging #561 into master will increase coverage by 2.46%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
+ Coverage   19.49%   21.95%   +2.46%     
==========================================
  Files          31       31              
  Lines        5068     5629     +561     
  Branches     1284     1617     +333     
==========================================
+ Hits          988     1236     +248     
- Misses       4057     4292     +235     
- Partials       23      101      +78     

Impacted Files	Coverage Δ
augur/export_v2.py	8.04% <0.00%> (+0.04%)	⬆️
augur/parse.py	9.45% <0.00%> (-1.15%)	⬇️
augur/ancestral.py	10.60% <0.00%> (-0.51%)	⬇️
augur/traits.py	8.06% <0.00%> (+0.09%)	⬆️
augur/refine.py	6.41% <0.00%> (+0.89%)	⬆️
augur/import_beast.py	9.27% <0.00%> (+2.52%)	⬆️
augur/align.py	50.88% <0.00%> (+4.26%)	⬆️
augur/clades.py	15.62% <0.00%> (+4.28%)	⬆️
augur/utils.py	32.80% <0.00%> (+5.17%)	⬆️
augur/filter.py	33.80% <0.00%> (+16.94%)	⬆️

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