Fix folder IDs to comply with api spec

[mvn23]

According to the subsonic api specification (see snippet below), MusicFolders should have an integer id. Currently, this is implemented in supysonic with an uuid. This PR fixes that. All folders now have an integer ID. This also fixes the issue where the music folder dropdown in Jamstash does not work.

<xs:complexType name="MusicFolder">
  <xs:attribute name="id" type="xs:int" use="required"/>
  <xs:attribute name="name" type="xs:string" use="optional"/>
</xs:complexType>

[codecov-io]

Codecov Report

Merging #165 into master will increase coverage by 1.46%. The diff coverage is 97.16%.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   88.98%   90.45%   +1.46%     
==========================================
  Files          33       33              
  Lines        2987     3037      +50     
==========================================
+ Hits         2658     2747      +89     
+ Misses        329      290      -39

Impacted Files	Coverage Δ
supysonic/api/annotation.py	100% <100%> (ø)	:arrow_up:
supysonic/api/browse.py	92% <100%> (ø)	:arrow_up:
supysonic/db.py	93.19% <100%> (+0.52%)	:arrow_up:
supysonic/api/albums_songs.py	93.54% <100%> (+0.28%)	:arrow_up:
supysonic/managers/folder.py	100% <100%> (ø)	:arrow_up:
supysonic/config.py	94.23% <83.33%> (+2.74%)	:arrow_up:
supysonic/api/media.py	93.6% <94.28%> (+13.2%)	:arrow_up:
supysonic/api/exceptions.py	83.78% <0%> (-12.17%)	:arrow_down:
supysonic/api/formatters.py	93.4% <0%> (+2.19%)	:arrow_up:
... and 2 more

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[mvn23]

Small update, the last review comment has been addressed. How would you prefer to deal with the other comments?

[spl0k]

Hi. Sorry for the late reply. The other comments were more general remarks than calls for action. I'll merge this PR soon.