[FEATURE] Don't auto-subscribe to new Issues/PRs

[kdumontnu]

This is a subset of #15743.

Gitea's current behavior appears to be to subscribe all watchers of a project to all new Issues/PRs in a project. In turn, they will get notifications for all comments/activity in those feeds.

I would suggest that this:
a. Is not the default case (matches GitHub behavior)
b. Is configurable either in user settings and/or gitea config settings

[truefolder]

I want to do this by adding the "Participating and mentions" option in Watch button. Like on a github.
I will need to add a drop-down list on the Watch button with three options - "Participating and mentions", "All activity" and "Ignore".
By default, the "Participating and mentions" option will be selected for all existing repositories (except for those that were created by you, "All activity" will be selected there), which will subscribe to notifications in PRs and issues if:

1. You created it
2. You left a comment there
3. You were mentioned

It turns out that I will have to work on both frontend and backend to add a drop-down list on the button and implement the option itself. Can I do this? And can I link this issue to the pull request?