Trac Notifications: improve tense for "subscribe" in trac_notifications_box_render()

[tyxla]

In trac_notifications_box_render(), the "subscribe" verb is used in present simple. But it makes sense to use present perfect tense there. These texts are displayed in the "Notifications" box of a particular ticket in Trac.

To clarify a bit better, here is a demo of how the notification texts appear right now:

• You are receiving notifications because you subscribe to the javascript focus.
• You are receiving notifications because you subscribe to the REST API component.
• You are receiving notifications because you subscribe to the 4.4 milestone.

After the change I'm suggesting, the texts will appear this way:

• You are receiving notifications because you have subscribed to the javascript focus.
• You are receiving notifications because you have subscribed to the REST API component.
• You are receiving notifications because you have subscribed to the 4.4 milestone.

[tyxla]

Improve tense for "subscribe" in trac_notifications_box_render()

[drewapicture]

In 1917:

Trac: Use present perfect tense for messages conveying focus, component, and milestone subscriptions.

Props tyxla.
Fixes #1254.