add API endpoint to remove an app's image

[tessus]

I have a few comments:

1. I don't know if the swagger comments are generated or if I have to manually add them before the function.
2. I am also not sure whether an error message makes sense, in case there is no image. IMO it should be a warning or info message, but not sure how to generate such a type.
3. I am not a UI person. I have ideas, but how to realize them with HTML/CSS is another story. It would take me a few days to figure these things out. But I suggest that there is an overlay image (an x) in the bottom right corner of the icon when hovered over the icon in the frontend. Upon click, the API is called. (The onclick js I can most likely do, but no idea how to do the overlay image.)

Update: I will also write the test cases as soon as the specs are finished for the API call. Tests added.

[codecov]

Codecov Report

Patch coverage: 76.47% and project coverage change: -0.11 ⚠️

Comparison is base (a37afce) 86.09% compared to head (690464c) 85.99%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #568      +/-   ##
==========================================
- Coverage   86.09%   85.99%   -0.11%     
==========================================
  Files          45       45              
  Lines        1568     1585      +17     
==========================================
+ Hits         1350     1363      +13     
- Misses        137      139       +2     
- Partials       81       83       +2     

Impacted Files	Coverage Δ
api/application.go	82.60% <75.00%> (-1.23%)	⬇️
router/router.go	88.11% <100.00%> (+0.11%)	⬆️

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[tessus]

Please let me know, if anything is still missing.