Add metadata field for WLE Phase I research themes

[alanorth]

In discussion and planning for #322 we decided that we should have a proper metadata field for WLE's phase I research themes, perhaps cg.identifier.wletheme. We need to tag all items in their research themes collections with the name of the collection (as that is the theme name):

• Decision Analysis and Information
• Ecosystems Service and Resilience
• Gender, Poverty and Institutions
• Land and Water Productivity
• Managing Resource Variability and Competing Use
• Recovering and Reusing Resources in Urbanized Ecosystems
• Regenerating Degraded Agricultural Ecosystems

We should do this at the same time as #322. After their website importer is switched to using the new metadata fields we can clean up and rename the collections for Phase II.

[alanorth]

After discussion (see #322) we decided to use one field for both Phase I and II research themes: cg.identifier.wletheme.