Power many sites from a single database (fixes #462)

[aptiko]

Checklist:

• [ ] All tests are passing
• [ ] I have added any necessary documentation and/or release notes
• [ ] I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• [ ] If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• [ ] I have removed all obsoleted code

[codecov[bot]]

Codecov Report

Merging #461 (931e65a) into master (57799d6) will increase coverage by 0.07%. The diff coverage is 98.36%.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
+ Coverage   97.74%   97.81%   +0.07%     
==========================================
  Files          19       20       +1     
  Lines        1505     1557      +52     
==========================================
+ Hits         1471     1523      +52     
  Misses         34       34              

Impacted Files	Coverage Δ
enhydris/admin/station.py	98.36% <96.00%> (-0.41%)	:arrow_down:
enhydris/api/serializers.py	95.69% <100.00%> (ø)
enhydris/auth.py	100.00% <100.00%> (ø)
enhydris/models.py	98.35% <100.00%> (+0.07%)	:arrow_up:
enhydris/views.py	100.00% <100.00%> (ø)
enhydris/views_common.py	94.44% <100.00%> (+0.80%)	:arrow_up:

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