Use angularJS for the "add set" form

[rolandgeider]

The form used to add sets to a workout could be implemented with angular (partially or completely). The current implementation has some problems, specially when the mobile version is used and the bandwidth is not the best, some of the django form management fields fail to update.

This could make some features, such as #154 potentially easier to integrate.

[rolandgeider]

Also, while the order of the exercises is the one selected, this is again depended on the speed at which the requests are processed and returned. This problem would be fixed if directly setting the order via the REST API.

[rolandgeider]

If this is implemented, #25 can be closed as well