feat: SlickVPN Support (#961)

- `internal/updater/html` package
- Add unit tests for slickvpn updating code
- Change shared html package to be more share-able
- Split html utilities in multiple files
- Fix processing .ovpn files with prefix space

Authored by @Rohaq 
Co-authored-by: Quentin McGaw <quentin.mcgaw@gmail.com>

internal/provider/slickvpn/openvpnconf.go

@@ -0,0 +1,30 @@
+package slickvpn
+
+import (
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/constants/openvpn"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+)
+
+func (p *Provider) OpenVPNConfig(connection models.Connection,
+	settings settings.OpenVPN) (lines []string) {
+	const pingSeconds = 10
+	const bufSize = 393216
+	providerSettings := utils.OpenVPNProviderSettings{
+		RemoteCertTLS: true,
+		AuthUserPass:  true,
+		Ciphers: []string{
+			openvpn.AES256cbc,
+		},
+		Ping:   pingSeconds,
+		SndBuf: bufSize,
+		RcvBuf: bufSize,
+		// Certificate found from https://www.slickvpn.com/tutorials/using-openvpn-configuration-files/
+		CA: "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", //nolint:lll
+		ExtraLines: []string{
+			"redirect-gateway",
+		},
+	}
+	return utils.OpenVPNConfig(providerSettings, connection, settings)
+}