Allow association and creation of activities from the context of a site

[pbrenton]

Currently the geospatial definition for activities is only possible from the context of an activity by selecting a site for an existing activity type/survey on a form.

Many users, particularly for the works project type, are requesting the ability to select existing activity types from the context of a site to create the activity/site association. eg. they want to be able to select an existing site or create a new site and then associate an activity with it, then either directly filling in the activity form at the time or saving it to complete activity form data at a later time.