add minimal ProxyFix support

[briantist]

related:

• Fix URL generation for scheme and self-referential URLs #29
• Some URLs don't have the right scheme even when setting preferred scheme #45
• Proto not set when running behind reverse proxy #27

While the PREFERRED_URL_SCHEME options adds some basic support, for actual reverse proxy use cases, the ProxyFix method suggested by @jcox10 is better. But it cannot be used in all circumstances; it must be used only when an actual proxy is in use, and it must be configured with the correct numbers for the headers, so it varies with each use case.

Being a Flask app, galactory should be run with a production WSGI server, and how to configure that server is up to each implementer, so I don't want to add too many options for this directly.

But this PR does add a basic way of configuring it. This method cannot be set with CLI/config options; rather it's done through arguments on the create_configured_app factory method, for example create_configured_app(proxy_fix=True, x_for=2, x_proto=2, x_port=2) where all the x_ options are optional and correspond to the options of the same name in the ProxyFix constructor.

This can be used with WSGI servers like gunicorn, so where before it may have been used like:

gunicorn 'galactory:create_configured_app'

it can now be used like:

gunicorn 'galactory:create_configured_app(proxy_fix=True, x_for=2)'

---

Using ProxyFix with x_proto (which defaults to 1) means not having to set PREFERRED_URL_SCHEME as long as the proxy sets the X-Forwarded-Proto header.

[codecov]

Codecov Report

Base: 56.83% // Head: 56.70% // Decreases project coverage by -0.13% ⚠️

Coverage data is based on head (95fd474) compared to base (fbb8e67).
Patch coverage: 40.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   56.83%   56.70%   -0.14%     
==========================================
  Files          26       26              
  Lines         943      947       +4     
==========================================
+ Hits          536      537       +1     
- Misses        407      410       +3     

Impacted Files	Coverage Δ
galactory/__init__.py	40.27% <40.00%> (-0.90%)	⬇️

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