Allow monogrammed objects to be parsed from the `arc` command line in "Reviewers" and similar fields
Summary:
Ref T10939. This allows the CLI to parse reviewers and subscribers like this:
```Reviewers: epriestley, O123 Some Package Name```
The rule goes:
- If a reviewer or subscriber starts with a monogram (like `X111`), just look that up and ignore everything until the next comma.
- Otherwise, split it on spaces and look up each part.
This means that these are valid:
```
alincoln htaft
alincoln, htaft
#a #b epriestley
O123 Some Package, epriestley, #b
```
I think the only real downside is that this:
```
O123 Some Package epriestley
```
...ignores the "epriestley" part. However, I don't expect users to be typing package monograms manually -- they just need to be representable by `arc land` and `arc diff --edit` and such. Those flows will always add commas and make the parse unambiguous.
Test Plan:
- Added test coverage.
- `amend --show`'d a revision with a package subscriber (this isn't currently possible to produce using the web UI, it came from a future change) and saw `Subscribers: O123 package name, usera, userb`.
- Updated a revision with a package subscriber.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10939
Differential Revision: https://secure.phabricator.com/D15911