Move to Oso 0.26

[lafrech]

There are still a couple of glitches but we need to move on.

For reference:

• Can't get shortcut "admin" rule to work osohq/oso#1536
• Oso 0.26 - Issue with entities appearing multiple times in a relation osohq/oso#1531

Also fixes Timeseries.get to use campaign scope rather than campaign (forgotten in #37).

[codecov]

Codecov Report

Merging #39 (e226ce4) into master (68d45c5) will increase coverage by 0.71%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   98.12%   98.83%   +0.71%     
==========================================
  Files          12       12              
  Lines         532      516      -16     
  Branches       74       69       -5     
==========================================
- Hits          522      510      -12     
+ Misses          5        2       -3     
+ Partials        5        4       -1     

Impacted Files	Coverage Δ
bemserver_core/authorization.py	97.26% <100.00%> (+1.70%)	⬆️
bemserver_core/model/timeseries.py	100.00% <100.00%> (ø)
bemserver_core/model/users.py	100.00% <100.00%> (ø)
bemserver_core/database.py	97.56% <0.00%> (+4.87%)	⬆️

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