fix: add cache wrapper for can_view_courses filter

[Ian2012]

This PR adds a cache wrapper for the can_view_courses function. The cache can be configured using the superset data_cache

It also removes the caching for the get_courses function such that people will get the latest courses at login.

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[itsjeyd]

Thanks for these changes @Ian2012!

[SoryRawyer]

Looks good! I tried memoized_func locally and it seemed to work as expected so I think this should help cut down on dashboard loading times.

[bmtcril]

Seems to do the right thing, I'm not sure about removing the get_courses memo though. I think it would still get called repeatedly for different field names, which is likely to happen a few times in quick succession in a dashboard load. Current cache time for all caches if 5 mins, that seems reasonable enough for getting fresh permissions. Since it was monitoring the access key it would get refreshed on a new login as well. What do you think?

[Ian2012]

We would have two caches, right?

Once for the get_courses and another one for the macro

[bmtcril]

Right, so if the macro got called with "course id" and then again with "course key" theoretically we wouldn't have to hit the LMS twice.

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