Refactor and improve discussion suggestions

[ruohola]

https://trello.com/c/CTkatfAG/880-show-own-school-first-in-discussion-suggestions

Main reason for this was to keep the most important content suggestions
first. E.g. the user's own school should always be the first school that
we're suggesting.

Changed

• Improve the relevancy of discussion suggestions.

[ruohola]

Tä eksplisiittinen tyypin tsekkaus on siks, että str on validi Iterable[str] tyyppi, mutta tässä se kärähtäis tosi pahasti: .order_by("grouped_ordering", *order_by) kun jos order_by olis vaikka "foobar" ni päädyttäis ajamaan query .order_by("grouped_ordering", "f", "o", "o", "b", "a", "r") joka olis lievästi sanottuna mielenkiinosta, ja se error message siitä ei olis kovin selkeä.

Pelkillä tyyppihinteillä ei oo mahdollista määritellä tätä rajotusta: python/typing#256

[codecov]

Codecov Report

Merging #288 (da7ec8d) into develop (0c257f1) will increase coverage by 0.18%.
The diff coverage is 90.47%.

@@             Coverage Diff             @@
##           develop     #288      +/-   ##
===========================================
+ Coverage    95.96%   96.15%   +0.18%     
===========================================
  Files           78       78              
  Lines         2629     2627       -2     
  Branches       169      167       -2     
===========================================
+ Hits          2523     2526       +3     
+ Misses          68       66       -2     
+ Partials        38       35       -3     

Impacted Files	Coverage Δ
skole/utils/shortcuts.py	95.34% <77.77%> (-4.66%)	⬇️
skole/schemas/comment.py	98.88% <100.00%> (+7.13%)	⬆️
skole/schemas/gdpr.py	96.82% <100.00%> (ø)
skole/schemas/suggestions.py	100.00% <100.00%> (ø)

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