Add subscribers count to issue

Description

I spend much time by search in issues subscribing them to watch discussion. But there is only metrics to find popular issues, popular ordering is nice but is build on comment count only, as far as i know. If i want to watch some issue, I subscribe it. It should be mail spamming, but i want to be informed. Everyone can subscribe, but how many of them write some message? So, i think that subscribers count metrics is more precise to show what is popular.

Also showing subcribers count in sidebar can be usefull. But only count, not list like participants.

(Include problem, use cases, benefits, and/or goals)

Proposal

Add subscribers count to sidebar of issue detail

Add ordering by subscribers count to filtering issues, maybe participant count order should helpfull

Links / references

this can be little addon to gitlab-ce#21747