Relax HTTP URL constraints on meta->implementation->source_url field.

[ml-evs]

As above, and from #258 & #260.

I also notice that we don't have any real tests for the info models directly, which I guess we should rectify at some point.

[codecov]

Codecov Report

Merging #262 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #262   +/-   ##
=======================================
  Coverage   89.32%   89.32%           
=======================================
  Files          54       54           
  Lines        2220     2220           
=======================================
  Hits         1983     1983           
  Misses        237      237           

Flag	Coverage Δ
#unittests	89.32% <100.00%> (ø)

Impacted Files	Coverage Δ
optimade/models/optimade_json.py	90.90% <100.00%> (ø)

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