On find friends page and associated json script, take a 'filter' param
and pass it through to both the search function and the pagination
builder.

The only valid options will be 'myinstitutions' (to filter results by
users who are in one or more of the logged-in user's institutions),
and 'all', to search everyone on the site.

The new parameter defaults to 'all' when the logged-in user is not in
any institutions.

Change-Id: I6cf829162bc0374ef63fa6848bd7f00755050bcc
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>