Add activity mount rules (#1026)

Activity Mount Rules associate a Mount Policy to an activity regex.

When recalling a file, the users can specify an activity (i.e. by using
filepath?activity=x). If specified, this activity will be used to select
a matching activity mount rule, if one exists. If more than one Activity
Mount Rule matches the activity, the one with highest retrieve priority
will be chosen.

Acitivity Mount Rules take precedence over Requester Mount Rules.