Split nginx config files and fix flower access config

[ilkka-ollakka]

Description

Create separated reverse_proxy nginx config for reverse-proxy setup out from production config. This needs documentation update.

Split nginx location configs to separate file, so same config can be included in development/production/reverse_proxy configs. This helps to keep location handling in sync between setups.

Also configure flower upstream so flower access works in all flows.

This also supersedes #3539 as it was not meaningfull to split changes out.

• Related Issue #
• Closes nginx reverse-proxy config needs to be updated #3279

What type of Pull Request is this?

• Bug Fix
• Enhancement
• Plumbing / Internals / Dependencies
• Refactor

Does this PR change settings or dependencies, or break something?

• This PR changes or adds default settings, configuration, or .env values
• This PR changes or adds dependencies
• This PR introduces other breaking changes

Details of breaking or configuration changes (if any of above checked)

Documentation

• New or amended documentation will be required if this PR is merged
• I have created a matching pull request in the Documentation repository
• I intend to create a matching pull request in the Documentation repository after this PR is merged

Tests

• My changes do not need new tests
• All tests I have added are passing
• I have written tests but need help to make them pass
• I have not written tests and need help to write them

[hughrun]

@bookwyrm-social/code-review I'm not super confident reviewing production configs but this looks like it could fix some longstanding issues with deployment vs dev environments so if someone can take a look at this, that would be great!

[mouse-reeve]

This works well for me, and it's a huge improvement to how we're handling nginx files -- thank you so much!