Respect original content when creating secrets (#24745)

Fix #24721.

Follow what GitHub does:
- Don't trim spaces for secrets.
- Newline should be `\n` instead of `\r\n`.

Did some tests with:

```yaml
name: secrets
on: push
jobs:
  show_secrets:
    runs-on: ubuntu-latest
    steps:
      - name: Dump secrets context
        run: echo '${{ toJSON(secrets) }}' | base64
```

`AAAAAA`:
```text
   AAAAAA
AAAAAA


```
`BBBBBB`:
```text



BBBBBB
BBBBBB   
```


On GitHub:

<img width="675" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/0ec60652-c2a3-47bb-9f9d-7e81665355a8">


On Gitea (before):

<img width="673" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/cce818bf-5edc-4656-86e1-2c81c304cdb2">

On Gitea (after):

<img width="673" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/0b3b15af-4d48-4bab-a334-4738a1b0eb4a">

routers/web/shared/secrets/secrets.go

@@ -5,6 +5,7 @@ package secrets
 
 import (
 	"net/http"
+	"strings"
 
 	"code.gitea.io/gitea/models/db"
 	secret_model "code.gitea.io/gitea/models/secret"
@@ -27,7 +28,15 @@ func SetSecretsContext(ctx *context.Context, ownerID, repoID int64) {
 func PerformSecretsPost(ctx *context.Context, ownerID, repoID int64, redirectURL string) {
 	form := web.GetForm(ctx).(*forms.AddSecretForm)
 
-	s, err := secret_model.InsertEncryptedSecret(ctx, ownerID, repoID, form.Title, form.Content)
+	content := form.Content
+	// Since the content is from a form which is a textarea, the line endings are \r\n.
+	// It's a standard behavior of HTML.
+	// But we want to store them as \n like what GitHub does.
+	// And users are unlikely to really need to keep the \r.
+	// Other than this, we should respect the original content, even leading or trailing spaces.
+	content = strings.ReplaceAll(content, "\r\n", "\n")
+
+	s, err := secret_model.InsertEncryptedSecret(ctx, ownerID, repoID, form.Title, content)
 	if err != nil {
 		log.Error("InsertEncryptedSecret: %v", err)
 		ctx.Flash.Error(ctx.Tr("secrets.creation.failed"))