cylc gui: Fix sorting of log files

Closes #1138

Changes the returned key from `_sort_key_func` so that it works from the last detected number onwards. This prevents numbers in a hostname being used for sorting in preference to the try number.

Tested as working.

A suite along these lines can be used to check the gui:

```
[scheduling]
    [[dependencies]]
        graph = "foo"
[runtime]
    [[foo]]
        command scripting = false
        retry delays = 5*0.1
        [[[remote]]]
            host = $(rose host-select --rank-method=random)
```

where rose host-select is picking from a number of alphanumeric hostnames (could be either servers of hostXXX format or perhaps hpc login nodes).