support custom .JS and .CSS files for custom banner messages (white-labeling) (#1781)

This PR introduces two new server configuration options, for providing a
custom .JS and .CSS file.
These can be used to show custom banner messages, add
environment-dependent signals, or simply a corporate logo.

### Motivation (what problem I try to solve)

I'm operating Woodpecker in multiple k8s clusters for different
environments.
When having multiple browser tabs open, I prefer strong indicators for
each environment.
E.g. a red "PROD" banner, or just a blue "QA" banner.
Also, we sometimes need to have the chance for maintenance, and instead
of broadcasting emails,
I prefer a banner message, stating something like: "Heads-up: there's a
planned downtime, next Friday, blabla...".

Also, I like to have the firm's logo visible, which makes Woodpecker
look more like an integral part of our platform.

### Implementation notes

* Two new config options are introduced ```WOODPECKER_CUSTOM_CSS_FILE```
and ```WOODPECKER_CUSTOM_JS_FILE```
* I've piggy-bagged the existing handler for assets, as it seemed to me
a minimally invasive approach
* the option along with an example is documented
* a simple unit test for the Gin-handler ensures some regression safety
* no extra dependencies are introduced

### Visual example

The documented example will look like this.

![Screenshot 2023-05-27 at 17 00
44](https://github.com/woodpecker-ci/woodpecker/assets/1189394/8940392e-463c-4651-a1eb-f017cd3cd64d)

### Areas of uncertainty 

This is my first contribution to Woodpecker and I tried my best to align
with your conventions.
That said, I found myself uncertain about these things and would be glad
about getting feedback.

* The handler tests are somewhat different than the other ones because I
wanted to keep them simple - I hope that still matches your coding
guidelines
* caching the page sometimes will let the browser not recognize changes
and a user must reload. I'm not fully into the details of how caching is
implemented and neither can judge if it's a real problem. Another pair
of eyes would be good.

cmd/server/flags.go

@@ -77,6 +77,16 @@ var flags = []cli.Flag{
 		Name:    "server-key",
 		Usage:   "server ssl key path",
 	},
+	&cli.StringFlag{
+		EnvVars: []string{"WOODPECKER_CUSTOM_CSS_FILE"},
+		Name:    "custom-css-file",
+		Usage:   "file path for the server to serve a custom .CSS file, used for customizing the UI",
+	},
+	&cli.StringFlag{
+		EnvVars: []string{"WOODPECKER_CUSTOM_JS_FILE"},
+		Name:    "custom-js-file",
+		Usage:   "file path for the server to serve a custom .JS file, used for customizing the UI",
+	},
 	&cli.StringFlag{
 		EnvVars: []string{"WOODPECKER_LETS_ENCRYPT_EMAIL"},
 		Name:    "lets-encrypt-email",

cmd/server/server.go

@@ -358,6 +358,8 @@ func setupEvilGlobals(c *cli.Context, v store.Store, f forge.Forge) {
 	server.Config.Server.StatusContextFormat = c.String("status-context-format")
 	server.Config.Server.SessionExpires = c.Duration("session-expires")
 	server.Config.Server.RootURL = strings.TrimSuffix(c.String("root-url"), "/")
+	server.Config.Server.CustomCSSFile = strings.TrimSpace(c.String("custom-css-file"))
+	server.Config.Server.CustomJsFile = strings.TrimSpace(c.String("custom-js-file"))
 	server.Config.Pipeline.Networks = c.StringSlice("network")
 	server.Config.Pipeline.Volumes = c.StringSlice("volume")
 	server.Config.Pipeline.Privileged = c.StringSlice("escalate")