Adguardforsafari: Standalone builds of AdGuard for Safari

[ameshkov]

We need to have two channels: beta and release

• [ ] Beta channel
• [ ] Release channel
• [ ] Add "How to release" section to the readme
• [ ] Update the readme and add links to standalone builds

Also, there's one more important thing about standalone builds, they must be notarized. See the details here:
https://github.com/AdguardTeam/AdguardForMac/issues/350

[iDjay]

How can we Beta Test version 1.3? Only got the key for the Mac version (2.0) but I prefer the safari extension

_{Sent with GitHawk}

[ameshkov]

@iDjay currently - there's no beta channel, but that's what we're trying to achieve in this issue -- establish standalone beta/release channels

[ameshkov]

@Mizzick auto-update seems to be working okay in the dev build.

There's one slight change that needs to be done: click on the notification should open https://github.com/AdguardTeam/AdGuardForSafari/releases