Add DATABASE_URL option to support MySQL and PostgreSQL backend

[my8100]

TODO: Update image to 'circleci/mysql:8' and set up PostgreSQL password on CircleCI.

[coveralls]

Pull Request Test Coverage Report for Build 155

• 82 of 143 (57.34%) changed or added relevant lines in 11 files are covered.
• 22 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.8%) to 87.322%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
scrapydweb/run.py	0	1	0.0%
scrapydweb/utils/check_app_config.py	9	10	90.0%
scrapydweb/vars.py	11	13	84.62%
scrapydweb/utils/setup_database.py	43	100	43.0%

Files with Coverage Reduction	New Missed Lines	%
scrapydweb/common.py	1	82.26%
scrapydweb/views/dashboard/jobs.py	2	91.72%
scrapydweb/run.py	19	0.0%

Totals
Change from base Build 141:	-1.8%
Covered Lines:	3320
Relevant Lines:	3802

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💛 - Coveralls

[codecov]

Codecov Report

Merging #76 into master will decrease coverage by 0.42%.
The diff coverage is 90.2%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   89.53%   89.11%   -0.43%     
==========================================
  Files          32       33       +1     
  Lines        3679     3802     +123     
==========================================
+ Hits         3294     3388      +94     
- Misses        385      414      +29

Impacted Files	Coverage Δ
scrapydweb/common.py	90.32% <ø> (+6.19%)	⬆️
scrapydweb/run.py	0% <0%> (-18.75%)	⬇️
scrapydweb/models.py	99.05% <100%> (ø)	⬆️
scrapydweb/views/system/settings.py	97.4% <100%> (+0.1%)	⬆️
scrapydweb/__init__.py	95.9% <100%> (ø)	⬆️
scrapydweb/views/overview/tasks.py	98.1% <100%> (ø)	⬆️
scrapydweb/vars.py	98.38% <100%> (+0.31%)	⬆️
scrapydweb/views/baseview.py	88.72% <100%> (+0.17%)	⬆️
scrapydweb/default_settings.py	100% <100%> (ø)	⬆️
scrapydweb/views/operations/execute_task.py	98.33% <100%> (ø)	⬆️
... and 5 more