Re-licensing DT as MIT

We are systematically re-licensing rstudio packages to use the MIT license, to make our package licenses as clear and permissive as possible. To do so, we need the approval of all copyright holders, which I have found by reviewing contributions from all all non-RStudio contributors. Would you permit us to re-license `DT` with the MIT license? If so, please comment "I agree" below. - [x] @wholmes105 - [x] @lrasmus - [x] @mtmorgan - [x] @rfhb - [x] @thomasrockhu-codecov - [x] @tyler-richardett - [x] @hafen - [x] @hdrab127 - [x] @jarauh - [x] @guoci - [ ] @gergness - [ ] @fbreitwieser - [x] @ecortens - [x] @andyquinterom - [x] @alexvpickering