Add first and last activity time fields to stackexchange index

[canasdiaz]

With the aim of knowing the active and inactive stackoverflow contributors, it is needed that the indexes are extended to have the first and last activity date of a contributor in the stackexchange index. When this is done a dashboard like the ones created for Git, Gerrit or Github can be used.

Requirements:

• extend the stackexchange index with first and last dates of each contributor
• integrate this with Mordred so it creates the needed aliases (if any)