Add pages to view watched repos and subscribed issues/PRs (#17156)

Adds GitHub-like pages to view watched repos and subscribed issues/PRs
This is my second try to fix this, but it is better than the first since
it doesn't uses a filter option which could be slow when accessing
`/issues` or `/pulls` and it shows both pulls and issues (the first try
is #17053).

Closes #16111 
Replaces and closes #17053


![Screenshot](https://user-images.githubusercontent.com/80460567/134782937-3112f7da-425a-45b6-9511-5c9695aee896.png)

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/base/head_navbar.tmpl

@@ -171,6 +171,10 @@
 							{{.locale.Tr "your_starred"}}
 						</a>
 					{{end}}
+					<a class="item" href="{{AppSubUrl}}/notifications/subscriptions">
+						{{svg "octicon-bell"}}
+						{{.locale.Tr "notification.subscriptions"}}<!-- Subscriptions -->
+					</a>
 					<a class="{{if .PageIsUserSettings}}active{{end}} item" href="{{AppSubUrl}}/user/settings">
 						{{svg "octicon-tools"}}
 						{{.locale.Tr "your_settings"}}<!-- Your settings -->