Add "types" tabs in Flags page

[ivanminutillo]

Bonfire implements polymorphic flags, which results in a list of different activities displayed on the flag feed available to the mods to act upon. Since different activities have different actions (eg. an activity can be hidden from feed or deleted, a user can be blocked), showing all of them on the same feed can become confusing and can add another layer of complexity for mods. The feed page would benefit from a tab to filter flagged activities by type, based on the enabled extensions.