InfoSource plugins

[joel99]

We need a new generic InfoSource plugin interface that provides targetting information for the server (aka either checkin or registration).

An InfoSource needs to provide two methods:

• options() should return filtering groups (e.g. grade, school) that can be used when requesting targets.
• filter(filterDict) will be given a dictionary of filtering groups with keys of filtergroups and values of arrays of accepted values (e.g. {grade: [9, 10, 11]}), and should return an array user objects that satisfy the filterGroups. This method should query the appropriate service for the actual information (checkin, registration). Once the data is received, process the information into a uniform format [i.e. plugin knows that a received UserEmail field is contains the email raw targetting information]

Once the interface is made, make a plugin for either plugin or registration (whichever is ready to serve the relevant info first).
Part of #13 epic.