Redirect supervisor to the right update page

[tymees]

During initial triage of the issue, I saw that actually fixing the issue was easier than to write it. So here we go.

This PR makes sure supervisors get the right update link on their 'decide page'. Previously, it would always redirect to the general one.

(I also moved the link-logic to python to keep the template sane).

[EdoStorm96]

Looks good. I approve, given the current state of the code. I am wondering though if it wouldn't be nicer to have a separate view for supervisor decisions and reviewer decisions. I was initially a bit confused looking at this template again. I suppose this would be a bit of work to do and its also beyond the scope of this issue.

[tymees]

It's the same view, as the underlying code is almost identical. So, taking care of the differences with some if-statements is more maintainable than having to remember to edit 2 views.

The same used to hold true for the template as well, but over the years we've been adding more and more differences. So yeah I agree, at this point, just loading a different template would make more sense.

Sounds like a great issue for when we're going to have to rewrite our templates anyway (updating to DSC 3)