Correct misnamed terms for lifetime to "period"

[riotbib]

This draft PR corrects terms for the lifetime that formerly where namend inconsistently "time" or "date". It introduces "period" as replacement.

Please note, this commit is not fully tested. I tried to, but the build failed. Please check carefully.

[riotbib]

Well, there we go.

As I forgot to mention before, scusi: This PR involves a breaking change downstream, because the form field time is changed to period.

[oxzi]

What's the status?

[riotbib]

Please (test beforehand and) merge.

[riotbib]

In hindsight, this was more nitpicking than improvment. Closing it.