switch to vue-cli v3

[maxpou]

Remaining work:

• fix tests
• test build
• check manifest.json + service worker
• check regressions with lighthouse
• adapt readme

Will close #41

[codecov]

Codecov Report

Merging #43 into master will increase coverage by 37.18%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #43       +/-   ##
===========================================
+ Coverage   36.49%   73.68%   +37.18%     
===========================================
  Files          19       19               
  Lines         137      114       -23     
  Branches       22       10       -12     
===========================================
+ Hits           50       84       +34     
+ Misses         87       30       -57

Impacted Files	Coverage Δ
src/components/RepoList.vue	100% <ø> (+100%)	⬆️
src/api/index.js	88.88% <100%> (ø)
src/components/MenuUserRepoList.vue	0% <0%> (ø)	⬆️
src/components/FileExplorer.vue	0% <0%> (ø)	⬆️
src/components/NotFound.vue	0% <0%> (ø)	⬆️
src/store/mutation-types.js	100% <0%> (ø)	⬆️
src/store/modules/navigator.js	100% <0%> (ø)	⬆️
src/store/index.js	100% <0%> (ø)	⬆️
src/store/modules/users.js	100% <0%> (ø)	⬆️
src/api/linkHeaderParser.js	100% <0%> (ø)	⬆️
... and 9 more

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