Update tale's icon field whenever imageId changes. Fixes #473

[Xarthisius]

Test case in #473.

[codecov]

Codecov Report

Merging #474 (9f81128) into master (bb2d57b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #474   +/-   ##
=======================================
  Coverage   92.11%   92.12%           
=======================================
  Files          52       52           
  Lines        3921     3922    +1     
=======================================
+ Hits         3612     3613    +1     
  Misses        309      309           

Impacted Files	Coverage Δ
server/rest/tale.py	95.12% <100.00%> (+0.01%)	⬆️

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[craig-willis]

Why have I never noticed this? Works for me.

[Xarthisius]

Why have I never noticed this? Works for me.

Judging by the amount of problems I found when I tried to switch base image on an existing Tale (whole-tale/gwvolman#143 whole-tale/gwvolman#142 and this) we haven't been really considering this workflow in our testing.