Rework config parsing

[jmattheis]

See the gotify-server.env.example for how it works. The new https://gotify.net/docs/config will just reference gotify-server.env.example, so it should be standalone and understandable.

• No yaml config anymore
• The list syntax changed from a yaml format to comma separated.
• Support loading settings from a file via _FILE
• Allow setting defining a custom path via $GOTIFY_CONFIG_FILE
• Supports gotify-server.env.local for local development, you can put GOTIFY_SERVER_PORT in there, and it works for the dev ui and server.

I've initially wanted to use a library (https://github.com/caarlos0/env) for the env parsing, but it didn't support _FILE natively. and manually implementing it doesn't seems to complex.

Fixes #366
Fixes #392

[codecov]

Codecov Report

❌ Patch coverage is 72.97297% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.35%. Comparing base (0e32e56) to head (2e71989).

Files with missing lines	Patch %	Lines
config/parse.go	60.65%	17 Missing and 7 partials ⚠️
config/file.go	62.74%	14 Missing and 5 partials ⚠️
app.go	0.00%	10 Missing ⚠️
config/error.go	0.00%	4 Missing ⚠️
config/config.go	97.40%	1 Missing and 1 partial ⚠️
api/oidc.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #967      +/-   ##
==========================================
- Coverage   74.46%   74.35%   -0.12%     
==========================================
  Files          62       66       +4     
  Lines        3008     3193     +185     
==========================================
+ Hits         2240     2374     +134     
- Misses        605      643      +38     
- Partials      163      176      +13     

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