Add the Molecular Mechanism Assessed controlled keyword to the experiment creator and editor

[bencap]

Although present in the database and set manually on certain experiments, this keyword is not exposed in the user interface. We should allow it to be edited on both the experiment creator and editor.

[EstelleDa]

@bencap Shall we add an Other option for this controlled keyword? We don't have it yet.

[bencap]

@EstelleDa Yeah, I think an Other option would be good.