remove extensions dir completly, create it on startup

[dni]

like we did for the data dir.
tested works great.
so no more unstaged git files when you are symlinking into your own extensions dir.

[codecov-commenter]

Codecov Report

Merging #1778 (c82aca5) into main (7e1f439) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

❗ Current head c82aca5 differs from pull request most recent head d68245c. Consider uploading reports for the commit d68245c to get more accurate results

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@@            Coverage Diff             @@
##             main    #1778      +/-   ##
==========================================
- Coverage   55.44%   55.44%   -0.01%     
==========================================
  Files          47       47              
  Lines        7250     7251       +1     
==========================================
  Hits         4020     4020              
- Misses       3230     3231       +1     

Impacted Files	Coverage Δ
lnbits/server.py	43.90% <0.00%> (-1.10%)	⬇️

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