Allow for individually selecting machines

[mdboom]

We currently have a drop down to select a single machine or all machines. We should instead have a check box for each machine so arbitrary collections of machines can be selected.

A multi-select drop down would be preferable, but that doesn't seem to be available, so I think we will be adding multiple checkboxes.

Cc @brandtbucher for the idea.

[mdboom]

Unfortunately, it turns out you can only have 10 options total, and we already have 8 (ignoring the machine field):

We could maybe have a string field that takes comma-separated values, but that's kind of a poor user interface...

[mdboom]

More context: https://github.com/orgs/community/discussions/8774

[mdboom]

The workarounds in the Github thread basically all amount to "write another UX, either commandline or web, to trigger jobs".

[brandtbucher]

Hm, bummer.

[mdboom]

We could accept a comma-separated list or something (maybe with some short names for each machine). Feels a little error-prone since we can't validate it quickly.

Or we could add different default sets of machines, e.g. rather than just "all", have "all, but just one Linux", "all 64-bit", "all ARM", "all Intel".