feat: granular settings for watched repo units (#10927)

Continuation of #10598
Closes #7254

Implement repo granular watch selection

I adopted a lot of the frontend from #10598 while redoing the entire back-end. I propose this new model:
- Remove the WatchMode enum.
- Add the WatchSelection struct, which represents the granular selection. Notice that there is no `watching` bool. I tried very hard to keep the structure as simple and redundancy-free as possible. Therefore, people not watching a repo at all either don't have a watch record or one with an entirely unselected WatchSelection struct.
- Add the WatchSource enum. It replaces the WatchMode enum and has a single purpose: determine whether a watch was explicitly or automatically initiated.

Notice that replacing this
```go
		And("`watch`.mode<>?", WatchModeDont).
```
with this is correct:
```go
		And(
			builder.Or(
				builder.Eq{"`watch`.watch_selection_issues": true},
				builder.Eq{"`watch`.watch_selection_pull_requests": true},
				builder.Eq{"`watch`.watch_selection_releases": true},
			),
		).
```
That's because there are four modes: dont, none, auto and normal. When `<>` with dont, we look for auto and normal, because there are no records with none. Therefore, the old code looks for records that indicate watching. The code I replaced this with does so, too, just more granular.

Also notice that I've prepared a future `user preset` in a few places. See below for a little more info on that.

## Next PR
I plan to continue working on this. I want to implement a `user preset` option. The user sets that `user preset` in her settings and may use them in any repo.
<details>
- rename account settings to account and notifications
- user preset (always use this preset for newly accessible repos (according to AutoWatchOnChanges and AutoWatchNewRepos))
</details>

## Further PRs
- make api able to granular watch
- move (email) notifications to new notifications tab

Co-authored-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10927
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

services/context/repo.go

@@ -643,7 +643,7 @@ func RepoAssignment(ctx *Context) context.CancelFunc {
 	}
 
 	if ctx.IsSigned {
-		ctx.Data["IsWatchingRepo"] = repo_model.IsWatching(ctx, ctx.Doer.ID, repo.ID)
+		ctx.Data["RepoWatchSelection"] = repo_model.GetWatchSelection(ctx, ctx.Doer.ID, repo.ID)
 		ctx.Data["IsStaringRepo"] = repo_model.IsStaring(ctx, ctx.Doer.ID, repo.ID)
 	}