Stats: Make Views & Visitors, Total Likes, and Total Comments cards data consistent with the Web

[staskus]

Calculate Views & Visitors, Total Likes, and Total Comments card data on Insights tab in a similar way as on the web, by excluding today. Also, remove ambiguity by renaming This week and Last week to Last 7-days and Previous 7-days which explains the numbers more accurately.

iOS issue: wordpress-mobile/WordPress-iOS#19882
Related issue: #17211

Views & Visitors

1. Calculate the last 7-days numbers same as the web, excluding today. Continue using the same endpoints, only calculate different data ranges.
2. Change the label This week to Last 7-days
3. Change the label Previous week to Previous 7-days
4. Change explanation labels from Your views this week are %@ higher than the previous week. to Your views in the last 7-days are %@ higher than the previous 7-days. (Same with the other similar labels)

Out of scope: unique weekly visitor numbers issue.
It may address #17450.

Total Likes

1. Calculate the last 7-days numbers same as the web, excluding today. Continue using the same endpoints, only calculate different data ranges.
2. Change the label Week to View more
3. Change explanations labels from ....higher than the previous week to ....higher than the previous 7-days (same withlower label)

Total Comments

1. Calculate the last 7-days numbers same as the web, excluding today. Continue using the same endpoints, only calculate different data ranges.
2. Change the label Week to View more
3. Change explanations labels from *%@%@ (%@%%)* higher than the previous week to *%@%@ (%@%%)* higher than the previous 7-days