Put concept exercises in status beta

[angelikatyborska]

Related issue`; #572

All but streams (mensch-aergere-dich-nicht) and dynamic-dispatch (rpn-calculator-output) are beta-ready IMO.

beta: This signifies active exercises that are new and which we would like feedback on. We show a beta label on the site for these exercise, with a Call To Action of "Please give us feedback."

Still wip:

• mensch-aergere-dich-nicht is waiting to be completely replaced by a new streams exercise [v3] Replace concept exercise for streams - mensch-aergere-dich-nicht #561
• rpn-calculator-output is waiting for a new dynamic-dispatch exercise that would come before it Implement new Concept Exercise: dynamic-dispatch #558

[neenjaw]

Agreed, the finishing touches you have done on them makes them quite polished I think