Make an unseen_count endpoint

[briehl]

Makes this endpoint - api/V2/notifications/unseen_count that just returns the total number of unseen notifications summed across all categories for a user's feed. Includes the global feed count as well.

Returns a structure like this:

{
    'unseen': {
        'global': ##,
        'user': ##
    }
}

[coveralls]

Pull Request Test Coverage Report for Build 213

• 19 of 19 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 90.411%

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Totals
Change from base Build 211:	0.07%
Covered Lines:	1518
Relevant Lines:	1679

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💛 - Coveralls