Load the local hostname's config, where possible

[nexy7574]

Description

This change mirrors some nice but niche behaviour from Element, wherein config.$HOSTNAME.json is tried before config.json.
This is very handy, at least for me, because I serve cinny from a multitude of domains, but all from the same dist directory. This means if you visit cinny from one of my domains that is for a different homeserver, you will get the config for only one of my homeservers.
An example of this is https://cinny.transgender.ing - the config.json is loaded, which is intended for https://cinny.nexy7574.co.uk. Element, however, nicely tries to load config.element.transgender.ing.json in the first case, meaning I can serve a different default login server.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

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