Track stale feature

[ptitFicus]

Removing unused flags is crucial to keep an healthy code base, however it may be hard to determine if a flag is still used or not.

Izanami could provide some hints about this using different methods:

• feature usage ( a feature that is never called is probably stale)
• feature value: a feature that always give the same value is probably stale. This one is trickier since feature evaluation can be performed client side, without having any info about resulting version in Izanami. We may add post eval hook client side that send evaluation data periodically to Izanami to clients and/or we could consider that some activation condition are stale-candidates (for instance on/off flag ir flag active for 100% users or flags with past activation date) while some other are not (fearure that activate for 50% users, script features, ...).

We could present "potentially stale" features in a dedicated UI and allow user to flag false positive / mute warning for qome time.