Simplify project configuration by allowing only one backend per project

[osma]

Fixes #218

[codecov]

Codecov Report

Merging #232 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
- Coverage   98.58%   98.47%   -0.11%     
==========================================
  Files          48       48              
  Lines        2120     2105      -15     
==========================================
- Hits         2090     2073      -17     
- Misses         30       32       +2

Impacted Files	Coverage Δ
annif/project.py	100% <100%> (ø)	⬆️
tests/test_project.py	100% <100%> (ø)	⬆️
annif/util.py	94.73% <0%> (-2.64%)	⬇️
annif/hit.py	94.11% <0%> (-1.18%)	⬇️

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