ThanksCount insight and CriteriaMatchInsightPluginParent

CriteriaMatchInsightPluginParent is a parent class for insights which look at some posts
    and track the change in how many posts match something over time.
InsightBaselineDAO::getMostRecentInsightBaseline() is added to support this.
ThanksCount is an implementation that looks at thankful posts.
ThanksCount copy tweaks

Capitalize network names in copy.
Change 1 time to once.
Split headline into two options.
Update CriteriaMatchInsightPluginParent and ThanksCount

Add docblock about parent class.
Add more variety to ThanksCount output.
Use getOccurrencesAdverb.
Thanks count - register insight, improve copy
Update weeklybest headline.

Specify that it was last week.
Switch all followercounthistory tests to use relative months and weeks.
Don't generate enough weeks for testing that a monthly is generated.  Oi.
Only show line chart for 3+ data points in ListMembership.
Update thankscount postmatcher to exclude self-replies and updated headline

Only show %username %posted X thanks when there is a thankee.