Customer portal improvements

[VixOrg]

First of all, thank you for this plugin - it is really useful for my setup :)
I have made some improvements to the customer portal plugin in my fork.
I believe these changes might be useful for other users of the plugin too, hence this pull request.

Here are the imrovements made:

• Add possibility to (optionally) display the Activity and Tags of the timesheet entry on the Details.
• Added possibility to display the monthly total amount just below the total hours, when the entry rates are not visible. This way admin can chose not to display the rates and price for each entry, while keeping the total amount visible to the clients.
• Added possibility to display the sub-totals of hours and amounts for each project (configurable)
• Added breadcrumbs for navigating from Project details back to the Customer details. Improved the Details button link to preserve the currently selected month and year in query params.

No backward compatibility breaking changes.