Ability to "select all" flow runs in a list for bulk deleting

[brandonreid]

Users have scenarios where they want to filter flow runs by certain parameters and delete all the runs that match. Currently they'd have to select each listed item one-by-one, which is grueling when there are many. Alternatively they could bulk delete via the API, but they'd prefer not to have to do that.

Let's provide them a way of selecting all listed runs:

When all have been selected:

If the user has manually selected an item or two:

[collincchoy]

Probably fixed by #2227?