Don't show share dialog if no incontext link

[seanh]

If the annotation received from the API contains no incontext link (as is the case with third-party annotations, for example) then don't render the annotation share dialog that contains the incontext link.

This is one half of the fix for #613, the other half is hypothesis/h#4714.

[codecov]

Codecov Report

Merging #618 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #618      +/-   ##
==========================================
+ Coverage   90.91%   90.95%   +0.03%     
==========================================
  Files         133      134       +1     
  Lines        5351     5374      +23     
  Branches      931      935       +4     
==========================================
+ Hits         4865     4888      +23     
  Misses        486      486

Impacted Files	Coverage Δ
src/sidebar/components/login-control.js	100% <0%> (ø)	⬆️
src/sidebar/filter/persona.js	100% <0%> (ø)
src/sidebar/components/annotation.js	88.73% <0%> (+0.05%)	⬆️
src/sidebar/components/hypothesis-app.js	90.36% <0%> (+0.11%)	⬆️
src/sidebar/components/group-list.js	95.65% <0%> (+0.19%)	⬆️
src/sidebar/components/annotation-header.js	88.23% <0%> (+0.35%)	⬆️

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[robertknight]

LGTM. Tested by removing both the "html" and "incontext" link generators in h/links.py in the "h" service.