Include notifications about observations by others in Notifications

[kueda]

This should be fairly light

Design needs:

• [x] Two tab layout
• [x] Where do we show new content indicators

Implementation

• [ ] Two tabs, one shows what's currently in Notifications (notifcaitons about your own obs), and the other shows notifications about other people's obs
• [ ] Each tab has an indicator showing whether there's unviewed content there
• [ ] Add new translation keys for notifications-user1-added-comment-to-observation-by-user2 and notifications-user1-added-identification-to-observation-by-user2
• [ ] Make sure the VOWORCON() Fluent function works, b/c Romance language translators will really need it for those two new keys
• [ ] Notifications icon in bottom tabs should show a new content indicator if there's new stuff on either tab
• [ ] Test the tabs with some longer text. The should ellipsize if there's overflow

[abhasinat]

Here are the new designs.

Key changes from what we have now:

• Tabs added to the header that is sticky to the top of the screen
• Each tab has a read and unread state (with and without a content indicator)
• The My observations tab shows all activity related to my observations
• The Others' observations tab shows all activity related to others' notifications