Add tabbed layout

[3j14]

This pull requests introduces a new layout that uses tabs to navigate the different steps of the measurement form or the two pages of the course administration.

To make the tabs work on mobile devices, they are not places within the title bar but instead below in the content:

For easier configuration, two classes TabNavigation and Tab have been introduced. These classes are subclasses of Renderable and their __str__ method returns renderable HTML.

Additionally, the package structure of gcampus/core/views has been changed and may require thorough review.

Note: There is now only one add-measurement endpoint. After adding the measurement, the title changes to "Edit Measurement #…".

Other fixes:

• Fix StructureIndexEditView to retrieve the correct instance from the current measurement.
• Return 404 error when accessing StructureIndexEditView or BiologicalParameterFormSetView for a measurement that is not of type FlowType.RUNNING

To-do:

• [x] Add documentation to new classes and methods
• [x] Add missing translations
• [x] Check for responsiveness

[nlwach]

I really like the new design of the tabs. Some minor things:

• The change from disabled and active for the tabs is IMO a bit too subtle
• The discard button is a dead link at the moment. This made me think. This button should discard the entire new measurement right? If that is the case I would also add it (the user could also just click any other link in on the left to navigate away from the create measurement section but I think it would make sense to have a discard button) in the first tab and maybe put a modal if the user really wants to discard this newly created measurement. For this we would then need to delete the newly created measurement after tab 1.
• I tested the responsiveness of the tabs on Safari for iPhone and Chrome mobile and everything seems to be working

[3j14]

@nlwach Thank you for your feedback!

Regarding the color of the disabled tab: Yes, I will change this and make the difference more noticeable.

As for discarding the measurement: Hm, good point. Obviously the button should do something. Making it delete the measurement is difficult because the same form is also displayed when editing the form. In that context, discard would not mean to delete the measurement but discard changes.

[nlwach]

@jnsdrtlf good point. I will implement this. How do you feel about having a modal to confirm to discard the changes?

[nlwach]

The discard button behaves now the following way: In the first tab the discard button redirects to the map view. In the other tabs the button redirects to the measurement detail page

[3j14]

@nlwach thanks for your changes and review! Merged now :)