Make persistent secret generation for `adminSecret`

**Is your feature request related to a problem? Please describe.**

This is related to #7 , although that issue seems a bit dated. Basically, when `createAdminSecret` is used (by default), a new secret seems that it will be generated on each release (I have yet to verify myself).

**Describe the solution you'd like**
In line with #73 , it is possible to generate values in such a way that they can be used consistently and preserved within Kubernetes (along with an optional user notification).

**Describe alternatives you've considered**
Putting the work onto the user to maintain / change these values (as is currently done)

**Additional context**

Depending on the outcome of #73, this would be fairly straightforward to implement within the helm chart. Wording and whether/how to notify in `NOTES.txt` (i.e. once for each value? Something less verbose?) will be important to consider.