Expand tilde in store path

[dominikschulz]

Fixes #801

[codecov]

Codecov Report

Merging #802 into master will increase coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #802      +/-   ##
==========================================
+ Coverage   64.61%   64.62%   +<.01%     
==========================================
  Files         170      170              
  Lines        9419     9425       +6     
==========================================
+ Hits         6086     6090       +4     
- Misses       2584     2585       +1     
- Partials      749      750       +1

Impacted Files	Coverage Δ
pkg/backend/storage.go	100% <ø> (ø)	⬆️
pkg/store/sub/storage.go	16.18% <100%> (ø)	⬆️
pkg/store/sub/store.go	53.15% <100%> (ø)	⬆️
pkg/backend/url.go	73.86% <66.67%> (-0.53%)	⬇️

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