Ensure the default environment is always dev (#4270)

[josephjclark]

Incoming branch for #4270

Closes #3954

Set up a branch for testing and tweaking

I've also lightly customised some help text in the Settings page

[josephjclark]

@theroinaochieng This is a nice small PR, based off a community contribution. Can you run a quick test (based on #4270) and merge?

I've tested locally and it all looks great to me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (93647d0) to head (ed64cf5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4285      +/-   ##
==========================================
+ Coverage   89.29%   89.34%   +0.04%     
==========================================
  Files         425      425              
  Lines       19899    19898       -1     
==========================================
+ Hits        17768    17777       +9     
+ Misses       2131     2121      -10     

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