Unattended updates

*This is not really a feature request, more of a conversation request to see if we (as the community) can come to a solution that satisfies everyone.*

**Is your feature request related to a problem? Please describe.**

The problem is that there were quite a lot of vulnerabilities found in ZeroNet earlier and we'll probably have many of them in the future. Every time a vuln is found, we have to broadcast the fix; unfortunately, what we are able to do now is definitely unsafe:

1. We can just publish this as a commit; looks good at the first glance but people most likely won't update and we can't count on the fact that no one reads commit history;
2. We can hide the fix in some merge commit or something; it's less likely to be noticed compared to 1. but this is definitely not a permanent solution;
3. We can publish the fix immediately and add a big "update right now" button to ZeroHello; this immediately makes the vulnerability existence (and thus the exploit) obvious, and the people who didn't update in time will have a big problem.

While some of these solutions are applicable right now, they aren't permanent and will definitely stop working well when new people come to ZeroNet.

**Describe the solution you'd like**

We could allow users to enable unattended updates, say, in /Config (it should probably be enabled by default because those who don't check /Config are most likely too non-techy to understand the consequences of the vulnerability). Some updates would be marked as "security". People who have unattended updates enabled would be updated automatically; those who don't will be shown a big red button on ZeroHello (probably also change `0` icon for visibility? These are implementation details though). The update would be cryptographically signed. Whether only nofish will have to sign it or someone else as well is another question; say, lowering the required signature count would make ZeroNet more vulnerable to a single person's sudden decision, and increasing the count can make the vulnerability known before the fix is signed.

**Describe alternatives you've considered**

The ones in the first section? Hehe.

**Additional context**

inb4: Quite a lot of well-known operating systems do that. For example, Ubuntu and Debian have `unattended-upgrades`, Windows (oh well) does that too. ZeroNet is cross-platform and also uses ZeroUpdate for updates, not a central repository, so we can't count on the OS-specific stuff.

@anoadragon453 @filips123 @blurHY