Generate resources based on template directories.

[bartfeenstra]

This fixes #155 and #448

[bartfeenstra]

Site generation is significantly slower than before, possibly due to more file I/O since we first copy all files to their destination, then render the tree which, for some renderers, skips their ability to render on copy.

[bartfeenstra]

The problem is that this completely prevents us from adding fallback templates later, using jinja2.environment.Environment.select_template().

[codecov]

Codecov Report

Attention: 40 lines in your changes are missing coverage. Please review.

Comparison is base (d975004) 94.66% compared to head (2004843) 93.20%.
Report is 559 commits behind head on 0.3.x.

❗ Current head 2004843 differs from pull request most recent head 1866bfe. Consider uploading reports for the commit 1866bfe to get more accurate results

Files	Patch %	Lines
betty/render.py	79.00%	21 Missing ⚠️
betty/fs.py	73.80%	11 Missing ⚠️
betty/sass.py	58.82%	7 Missing ⚠️
betty/jinja2.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            0.3.x     #458      +/-   ##
==========================================
- Coverage   94.66%   93.20%   -1.47%     
==========================================
  Files          31       31              
  Lines        2381     2471      +90     
==========================================
+ Hits         2254     2303      +49     
- Misses        127      168      +41     

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