Add proper date selector for activity view

[ErikBjare]

To make it easier to navigate further back.

Would be nice to only be able to select dates which have data for them.

[johan-bjareholt]

Was looking at trying to fix this for the dev/vue2 branch which uses bootstrap4, but apparently bootstrap4 is in alpha and does not have a datepicker yet...

And apparently the HTML5 datepicker isn't implemented in firefox yet (Works, but not enabled by default yet http://caniuse.com/#feat=input-datetime)

[ErikBjare]

I don't mind waiting a bit then. Would definitely prefer the HTML5 datepicker.

[ErikBjare]

I'm not sure if going with the HTML5 datepicker is the right way to go since there is no way to mark which dates have data and which don't (only min and max date constraints, although that might be good enough). Dates also tend to appear in the mm/dd/yyyy format and other non-sane not-ISO8601 formats.

Still not a high priority, but something to consider.

[johan-bjareholt]

It was trivial, so added it in this PR, better than before at least

#63

[ErikBjare]

Closing this for now, #63 seems good enough.