feat: Local environment mode / integration testing setup

[matheus23]

This implements some plans from https://talk.fission.codes/t/meeting-notes-control-panel-needs-from-fission-server-roadmap

When you set this in settings.toml for the server:

[server]
environment = "local"

And start the fission server binary as normal, then you won't need to provide an API key for mailgun, but it'll still work.

Verification codes will be logged & sent on the websocket relay (with the email being the topic name).
This allows programatic integration testing without requiring mailgun access.

[codecov]

Codecov Report

Merging #200 (2dee5e1) into main (add13f7) will decrease coverage by 0.48%.
The diff coverage is 11.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #200      +/-   ##
==========================================
- Coverage   18.63%   18.16%   -0.48%     
==========================================
  Files          62       63       +1     
  Lines        2522     2588      +66     
  Branches     1556     1599      +43     
==========================================
  Hits          470      470              
- Misses       1428     1492      +64     
- Partials      624      626       +2     

Files	Coverage Δ
fission-server/src/extract/authority.rs	29.16% <ø> (ø)
fission-server/src/lib.rs	100.00% <ø> (ø)
fission-server/src/models/account.rs	44.00% <ø> (ø)
fission-server/src/models/email_verification.rs	22.58% <ø> (+11.10%)	⬆️
fission-server/src/models/volume.rs	0.00% <ø> (ø)
fission-server/src/routes/account.rs	17.64% <ø> (ø)
fission-server/src/routes/auth.rs	22.22% <ø> (ø)
fission-server/src/routes/capability_indexing.rs	25.00% <ø> (ø)
fission-server/src/routes/doh.rs	19.23% <ø> (ø)
fission-server/src/routes/health.rs	51.85% <ø> (ø)
... and 11 more