Pull request: all: imp dev version handling

Merge in DNS/adguard-home from fix-version to master

Squashed commit of the following:

commit ecef63315fb49ae33b4c3f13c0e0be0668340e2b
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jan 22 18:59:17 2021 +0300

    updater: imp tests

commit f5243918567430e467c44a48e45169db4560b58b
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jan 22 18:48:10 2021 +0300

    all: imp dev version handling

internal/version/version.go

@@ -13,12 +13,20 @@ import (
 // TODO(a.garipov): Find out if we can get GOARM and GOMIPS values the same way
 // we can GOARCH and GOOS.
 var (
-	channel string
+	channel string = ChannelDevelopment
 	goarm   string
 	gomips  string
 	version string
 )
 
+// Channel constants.
+const (
+	ChannelDevelopment = "development"
+	ChannelEdge        = "edge"
+	ChannelBeta        = "beta"
+	ChannelRelease     = "release"
+)
+
 // Channel returns the current AdGuard Home release channel.
 func Channel() (v string) {
 	return channel