#426 gateway - Add UI customization part

[Gaetanbrl]

ref #426

This PR :

• adds the Gateway UI customization resources and configuration.
• change default .css to use georchestra's main color (# georchestraStylesheet=/public/stylesheet.css)

See the documentation for details:
https://docs.georchestra.org/gateway/en/latest/user_guide/ui_customization/?h=thy#main-principles

The files originate from:
https://github.com/georchestra/georchestra-gateway/tree/main/gateway/src/main/resources

[f-necas]

I don't think we need to import /static/ from the upstream repo. We can let them inside and provide a good documentation of how to use spring.web.resources.static-locations.
We can point to upstream repo in order to show what's already in the classpath.

I may also rename html files in login.html.example to avoid confusion about where are files fetched.

[f-necas]

As the comment I wrote in : georchestra/georchestra-gateway#235 (review)

You can set up static-locations: file:/etc/georchestra/gateway/static/, classpath:/static/. With this line, resources willl be fetched from datadir first and then fallback into classpath:/static/

[f-necas]

Suggested change

	# static-locations: file:${georchestra.datadir}/gateway/templates/static/
	# In order to import your resources, create a folder /etc/georchestra/gateway/static/ and put them inside. e.g. If you create /etc/georchestra/gateway/static/img/myimg.png, your resource will be available on <your domain>/img/myimg.png
	# static-locations: file:/etc/georchestra/gateway/static/, classpath:/static/

[f-necas]

Not mandatory by default it's /.

It just change the path in url with prefixing it, but folder hierarchy may resolve this. By now, resources are fetched only from classpath:/static/ and the folder hirearchy inside is everything under /login.

So by default, we can resolve static resources with the url /login/error.css or /login/img/default.png

If you set # static-path-pattern: /static/**, urls will be /static/login/error.css or /static/login/img/default.png.

[f-necas]

Suggested change

	# static-path-pattern: /static/**
	# Change this line if you want to set a "base path" to your static resources
	# static-path-pattern: /static/** # <- your resource will be available with a /static/ prefix