Change up how `public/` is structured

Currently many random files are served from public/. Some have a clear purpose and are obviously of importance, others are less so.

As part of our infra-rebuild we'd like to minimize/clean out this folder.

You can review the current contents here.

This issue will be updated with plans for each item.

Item	Last Update	Action	Reason
`apidoc*/`	1 year ago	Delete	Document how to generate.
`apipie-cache/`	2 years ago	Delete	Other API docs exist that are more up to date.
`app/`	4 years ago	Delete	It doesn't seem like this is used?
`crossdomain.xml`	4 years ago	Keep	Needs to be in webroot.
`(icons\|marker).zip`	3 years ago	Keep	I could see them moving to `public/assets/` but there is no real need.
`map-test.html`	1 year ago	Delete	@lennerd has explicitly stated this is fine.
`robots.txt`	3 years ago	Keep	This needs to be in the webroot.
`google$BLAHBLAHBLAH.html`	~5 years ago	Delete	Should be able to be deleted (https://github.com/sozialhelden/wheelmap/issues/424#issuecomment-252178901)

Item	Last Update	Action	Reason
`bounds/`	~5 years ago	Delete	This seems like an old datadump of some kind? No references to this directory in code.
`javacripts/i18n.js`	6 months ago	Move	I see no reason this shouldn't be in `public/assets`.
`system/`	5 years	Keep	Contains maint static
`(404\|422\|500).html`	6 years ago	Move	Place in `system/` with the maint static.
`theme/default/`	4 years ago	Delete	This appears to be unused. If it is used it should be in `assets/`
`favicon.ico`	4 years ago	Keep	Move to `assets/`, needs a change in headers.

Item	Last Update	Action	Reason
`marker/`	3 years ago	Delete	Loading the site fetches these marker related assets. Nothing in here is included. Do mobile apps use them? They should use `marker.zip` as that is the only reason it exists.

sozialhelden / wheelmap-classic