Fix timestamps not sorting in datatables

Timestamps retrieved from the API were always converted to a browser
local format. The format specified for moment.js added in
5160eff2 did not work because of this.

Additionally, the format specified used `dd` which looks for two letter
days, such as "Mo", "Tu", "We", etc. Furthermore, `mm` is used for
minutes, not months.

Because the locale formatted datetime can vary a lot, it is not easy to
get this into moment.js to enable the sorting of datetimes in the
datatables. In other words, there is no conversion from an
`Intl.DateTimeFormat` specifier string to moment.js. Adding many
`$.fn.dataTable.moment(format);` with different `format`s is not useful.

I have fixed this rewriting how the timestamps from the API are added
to the tables. It still uses the locale of the browser, because not
everyone wants to use ISO 8601, but no longer requires moment.js (which
has been removed).

Two data attributes are added to the `td`s of the timestamps:
- `data-order`
- `data-sort`

The values of these are the timestamps as returned by the server, which
are very easily sorted (as they are just UNIX timestamps). Then, when
creating the cell in the table, it will be converted to what the locale
of the browser specified (this has not changed).