Use enableFeature prefix for all feature flag config parameters

[willdurand]

In order to better identify the feature flags among all the config parameters, let's adopt the enableFeature prefix for these config params. The idea of this issue is to rename all parameters.

For QA:

• static themes support should be enabled on all envs
• the new install button should be available in -dev and -stage on AMO, in -dev only on Disco Pane
• inline add-on review should be in -dev only (AMO)
• the experience survey should be disabled entirely
• add-on recommendations (AMO detail page) should be enabled

[AlexandraMoga]

Verified fixed on -dev stage and prod with FF62, Win10x64