Parameterise Institution - step 1 - seeds

[baob]

Part of https://github.com/swapmyvote/swapmyvote/issues/1018.

Need to at least

• move 'uk parliament' related seeds to a subdirectory
• extract 'uk parliament' parties to yaml file (presently Party model)
• add seeds for US 'parties' (actually, presidential candidates) and constituencies (states)
• add a SMV_INSTITUTION env variable
• make it easy to seed for 'uk parliament' or 'US president' by switching the SMV_INSTITUTION variable.