create member initiatives tab/subpage

[KristijanArmeni]

This is in part a brainstorm issue.

See whether we can set up a member initiatives subpage. Perhaps a form that can be filled in by the 'proposer' via the website (like the newsletter form).

Or just a separate subpage where it is explained what member initiatives are and which are already running (like a portfolio or sth).

[KristijanArmeni]

With #55 and #57, we also created the initiatives tab. Not a form, but simply a starting page linking to active initiatives for now.

For now only listing the reproducibiliTea as initiative. The 'initiatives' subpage could do with some extra/brief information though.